This is a lot of note organizing after doing some recent experiments with a Christian Japan from the 867 start date. This is pre-Silk & Silver DLC, which is going to significantly rework the game's economy and may render some of this obsolete. I wanted to get it out of my head.
Did I mention this is long? Feel free to grab a drink and relax. Add your own experience if you please. Errors are inevitable.
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Why Christian Japan?
Japan offers a variety of unique mechanics to help get a new Christian faith started in the far east.
House blocks give defensive allies that normally wouldn't agree to marriage alliances, and the Kampaku is more able than most to defend. Japan's relative isolation means you won't be warred by any nearby coalitions or religious wars. The main threat early on is schemes, except its easy enough to mitigate those.
Japanese manors are also excellent for surviving skill-based checks in events (like pilgrimages) or schemes in religiously hostile zones. Each family focus unlocks a different unique building that gives a +1-per-level buff to a skill, but you can benefit from previous buildings if you later change. The tribal-era manor alone can provide +15 intrigue to help defend against hostile schemes... or pursue them.
Imperial policy laws provide a variety of contextual buffs and tradeoffs. Most relevant to a religion-strategy is Manor Reform, which provides -15% building cost and build time for when you start building out and upgrading temples in earnest.
Finally, Japanese pilgrimages to major holy sites abroad are something. Ever gone from Cathay to Ireland? You can, and while that trip will probably take 5-6 years, you can legitimately earn nearly a dozen perks with the trip just from all the POIs and such along the way. A new ruler who just assumed the throne, pacified the vassals with coronations and a tour, and then takes a long pilgrimage that can instantly jump them up decades worth of points.
Why Christian Japan?
Once it's set up, the Christian mechanics offer some useful tools that Japan might otherwise not have.
Theocratic-faith leased temples are an excellent money maker for Japan's lower domain cap rulers. Counts who could normally only hold one county were able to get more from temples than from cities. Christian theocrats also bring access to the brewing hall, a gold-and-development building that offers more econ benefit that you can tax. They also now pay (low) taxes to religious superiors, which can be taxed by you outside of normal tax channels.
Christianity breaks out from the religious hug-box that is the far east, where everyone is too accepted to holy war their most neighbors. There are some exceptionally tempting targets for Kingdom-scale holy wars in the region, especially during the Chinese crackups. It is possible to take the entire Korean peninsula in a single war if you wait a century, which is about what you have to do anyway for the normal tech development to get the external wars law.
Faith hostility paired with the new puppet system can invert the personal expansion prospect to exploit defensive war mechanics to bolster religious fervor instead. By puppeting a foreign ruler and having them declare war on a house block, any ruler could exploit the defensive coalition for easy defender money. Declaring a holy war doesn't even require being the target to protect a co-religionist, and gives the defending faith twice the fervor cost by the attacker. That's an easy 10 fervor if you provoke a Kingdom-scale holy war, easily covering the Fervor cost of your own holy wars.
Christian ecclesiarches can manipulate claims and claimants in a way most can't. They can instantly de jure drift subordinate titles, even Kingdoms, at a cost of piety and prestige, substantially increasing taxes and reducing faction risk. They offer another way to force a succession turnover on vassals, especially if done via puppet. They can demand vows of chastity, strangling a house's fertility potential so a once-large house becomes a fragment of itself.
Why Japanese Christians?
Japan's unique Yamato culture has a surprising number of synergies with Christianity.
Ephemeral Grace provides stress lost bonuses to several Christian tenet virtues, namely Temperate, Compassionate, Calm, and Patient. It buffs poet, a trait you can pick up from Japanese festivals, which offers a sin-free way to stress-attack people, or provide piety for Christian interactions.
Yashima provides a -10% holding cost in mountains and forests, i.e. most of Japan and your expansion areas, making for easier temple spam. It greatly reduces the traveling over seas, hills, and mountains, making the super-long-range pilgrimages that much safer.
Dynastic Pragmatism provides a culture-level access to polygamy, allowing Japanese to be the only polygamists when others who convert will be monogamists. This is a massive advantage, especially in admin systems where family size drives influence.
Fragile Peace allows gives prowess buffs to martial education and lets even unmarried female family, such as nun-daughters, serve as knights or commanders in defensive wars that you can instigate via puppetry to exploit block confederations.
Warrior Monks allows clergy to serve as commanders, saving a doctrine slot for your custom rite, even as it gives major prowess buffs for learning education and theological-related traits. It also gives Ecclesiastical vassals prowess for holding monasteries in their temples.
Warrior Monks also gives access to the Warrior Monk MAA. These are mid-tier spearmen who are paid in piety rather than gold, which is easily earned if fighting defensive wars. They are exceptionally cost effective, and play a good support to a smaller number of premium mounted samurai that you don't want to waste sieging down settlements.
Why Ritsuryo Japan?
Ritsuryo, specifically, has a few more synergies for Christianity than feudal-with-estate Soryo.
Influence-based succession without a state rite means that a ruler didn't have to be religiously-popular to become Emperor, they just need to be able to out-bid the difference. Once Kampaku, vassals who don't convert can be retired or revoked tyranny-free. This can be critical to get over the initial hump of being an unpopular / short-reign-penalty / heretical ruler, since an initially massive block faction can be more quickly dismantled than a Soryo feudal block.
Influence also allows Japan to make better use of vassalized meritocratic realms, which are possible via holy war if you take a pluralistic doctrine. Ritsuryo doesn't have duchy-level titular MAA, but the influence used by Ritsuryo can be used to call forth meritocratic duchy MAA. This more than offsets initial Soryo MAA advantages over time.
Influence also gives Ritsuryo a slight edge in various skill-scaling manor upgrades. While most manor upgrades scale by fame, some only scale by level of influence, which Soryo can't compete in. This is how early Ritusuryo can get +2 per level of prestige, and +1 per level of influence, for up to +15 intrigue on any character.
There are especially high-synergy Christian tenets for that provide more benefit the more single-county counts you have. To Work Is To Pray gives the develop capital decision. Love Thy Neighbor provides major buffs to counties with +60 popular opinion, and a capital provides +20. Scholasticism isn't county-scaling, but excels for long-term tech growth, suited for an often-isolationist Japan.
I think- but wasn't able to check for unstated conditions before I lost the save- that Christianity's ecclesiastical mechanics allows an (eventual) way to create Duchy titles within Japan without going Soryo. Vassals of a sufficient piety level can ask archdiocese for a title that hasn't been made yet. This would mean Ritsuryo Japan could eventually benefit from duchy buildings and far less vassal count bloat, allowing it to expand far more.
Ritsuryo Japan's end-game is also conducive for managing faith fervor. While Soryo gets the fancy shogunate, Ritsuryo can prevent Soryu conversions by reaching (and maintaining) max crown authority. This prevents both internal and external wars, including holy wars. Since external wars are impossible until you have access to the laws to forbid any vassal wars at all, vassals of your faith won't ruin the fervor by over-waging war against weak tribals.
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The Core Strategy
The most consistent way I found to start the conversion process was a top-down approach as the Kampaku. While you can survive as a Christian admin count, the ability to get vassals to convert with you for free (and a lot of free piety) requires you, well, not be a Christian first.
The key steps are to do a number of long pilgrimages to India for piety and POI XP for perks while doing religious schemes on nearby Nestorians, get a level 2 holy legend complete as Kamapaku, and then to do a top-down conversion process by converting to Carolingian Christianity specifically before making a branching rite from there. Longer term, church construction keeps fervor high and helps lock-in a powerful base development buff.
Early Life
The following guide works more or less the same for anyone between the 867 ceremonial emperor, and a no-family custom count. Any count can generate gold for travel and manor upgrades, and influence for new positions, by resigning from a county. Resign after doing a governorship event, and sink the influence into a county held by a elderly vassal at deaths door, rinse and repeat, eventually rigging elections for your house to succeed your abandoned governorships. The ceremonial emperor and Kampaku can both the first 2 dynastic legacies of the Activities tree, giving 50% discount to pilgrimages among other buffs.
The opening strategy is to use frequent travel to get travel POI lifestyle experience for early high-payoff perks, piety for eventual reformation, and prestige for culture reform.
Start with a silk road market visit to China, as it can get enough stewardship XP for Golden obligations, and can refund the travel cost if you go for gold rather than the tech unlock. (You won't unlock the tech any time soon, and there will be later chances with better odds to unlock.) Pilgrimages to India provide tons of piety, as well as POIs all across southeast asia and indonesia. 2 or 3 pilgrimages is all you should need, and offer enough paths to hit all the major POIs.
Use the travel lifestyle, 50 prestige per POI, as your opening. The prestige will race you through levels of fame manor bonuses, and cover eventual cultural reformation. The inspection tree unlocks inspections, which can provide net gold, but also culminates in +1 maximum domain limit. This is distinct from +1 domain, as it mitigates the Japanese penalty and lets you go from an initial 1 domain max to 3 when the manor is upgraded to tier 2. Consider picking the opening perks for travel safety and private armies from the other two trees first, though, to mitigate the danger (and time) of long pilgrimages.
For your manor, upgrade the main building for a second domain limit as soon as feasible, but otherwise focus on building the buildings unlocked by family attributes. These provide powerful stat scaling benefits that help you be dominant in county elections, pass event checks, or nearly guarantee scheme success. Plan to pay for upgrades via blackmail gold, preferably when the Kampaku has shifted laws to not increase the manor development cost by 25%. The level 2 building upgrades do not provide additional building slots or major upgrades, so it's strictly optimal to just build level 1 and then upgrade far later when your economy is much stronger. When looking to shift family attributes, don't pay the cost yourself, just join the corresponding block and wait for the block leader to bribe you to the change.
Your first two manor slots are recommended to be family archive (service) and family brewery (prosperity). Both provide +1 stewardship per level of influence or fame, a key stat for passing inspections and stewardship checks that provide the most early-game gold infusions. Family archive also provides sub-building access to the mentor governorship scheme, or a tutoring buff for better childhood education. The brewery and its sumo sub-building provides the best early game revenue to cover court positions. Ideally start with the family archive (service block), and then swap to the prosperity block for longer-term economic benefits until you can upgrade your manor. Later swap to the a different family attribute, and let that crash the prosperity block to reduce a long-term threat.
After you upgrade the manor, your next two slots should be the guard house (determination) and family shrine (humility). The guard house, one of its upgrades, and the servants quarter option for the manor provide +3 scaling intrigue, and is key for when you start running constant kidnap or hook schemes. The family shrine provides learning scaling, temple discounts, and a faith creation/reformation discount for when you customize your faith.
The last but most important step is to find one of the in-range Nestorian characters and study their rite. This is what will let you eventually jump to christianity as an informed convert, rather than a foolish convert with huge stat / stress / piety penalties. There is a small chance you'll be able to find a Coptic wanderer in range as well.
This period transitions to the Kampaku period when, well, you become Kampaku via election. Election to Ritsuryo elections become much easier with higher stat scaling from your Manor.
Kampaku Period
In this period you set conditions for the conversion process.
First, you'll want a lot of gold. Use your use your 3-domain limit to get the most profitable counties- likely the gold mines- for yourself. As emperor do a Grand Taxation Tour. You'll need to stabilize your realm, have a coronation, start generating artifacts, and keep upgrading your manor.
Part of the gold pursuit is vassal development and pre-emptive disarmament. This section's specifics will change in Silk and Silver, but Ritsuryo government (at time of writing) has +30% to gold income, but base income is trash due to lack of econ buildings. Give vassal directives to every vassal to build econ buildings, not fortifications. Use gold to fill every empty building slot with a harbor before the AI builds less effective buildings, starting with direct vassals but then their temples and eventually cities. Whenever you revoke a title for any reason, make cities the default county capital so the next governor pays more gold and has fewer levies, and replace the castle walls with more profitable.
You'll also want to do some immediate cultural reform. Use the prestige from traveling to remove the 'Inward Perfection' tradition. This is a mostly-bad tradition that ruins your marriage game, which is big for spreading Christianity through dynastic marriages later. Replace it with something preferably economic-related. Silk & Silver may change the following, but at present I'd recommend the Rice Cultivators traditions, which unlocks rice paddies and manors in hills and jungles. These are not only excellent gold and development buildings in their own right, but which scale further with Ritsuryo government gold income buffs, but they offer synergies for future expansion and reformations covered later. (Again- unclear how S&S will shape this.)
This is the period where you will swap from traveler lifestyle to your next. Use learning lifestyle and the pilgrimage POI to get the apostate and prophet perks if you don't have enough POI/wiseman experience. The +1 tenet slot from theocratic is also very good.
Once your learning tree essentials are covered, the next tree is the intrigue tree. Unlocking hook fabrication and abductions is key to starting recruiting / getting ward acceptance for conversions by education, and dark insights plus no piety from torture can provide both stabilizing dread and more stat scaling for safety and scheming. It's possible to get +20 intrigue between manor buffs and Dark Insights, which easily allows you to abduct even Chinese kings during the warring states. Maxing the Schemer tree is an easy way to get foreign puppets, which can do major things like compelling the King of Silla to release the dejure Korean kingdoms before he absorbs them, or keeping the partition of China going forever.
You will really want to do a holy legend for the evangelize the realm conversion decision, which will convert a majority of the counties that don't automatically shift when you convert to Christianity. You should get the holy legend seed from your pilgrimages. Japan is actually really, really good at pumping out tier-2 legends, because you have enough vassal counts to ask them directly to spread your legend and get from level 1 to 2. You can't conclude the legend until year 5, and costs scale with spread, so just let it spread naturally for the first few years before using a few courtiers to spread it domestically. Don't worry about tier 3. Do plan on building the legend artifact eventually, because it's a fine option to consecrate for an eventual holy site relic.
Ensure that the house block situation is to your favor. When you convert- especially if you convert shortly after assuming power- you will be at high risk for house block factions. If you have a lot of family, use Kampaku interactions to revoke and retire anyone who would revolt. Over the long-term the dominant play in Ritsuryo is to replace everyone with your family so that any faction you join crashes under its own weight, even if you have the right family aspiration. You use this to crush house blocks before a member can convert to Soryo, so that they are isolated and easy to arrest and reverse.
This period ends at the conversion process. You want around 6k piety on-hand for this, a completed holy legend, and the learning perks for both easy rite conversion (Scholar tree) and full access to custom rite building (Theocrat tree).
Conversion Process
This is your basic order of operations to make the switch in two main steps: converting to Carolingian Christianity, and then your custom rite.
Why Carolingian Christianity? Because it starts with Fervent Proselytism, which gives you more piety when you convert counties or landed rulers. This applies to the rulers who convert along with you when you adopt the rite. This provides several thousand more piety from the counties, counts, house heads, and their family that convert with you.
Once you convert, immediately make your own custom rite. I'll explain my pitch shortly, but the key point is that you want to stay relatively low divergence with the Roman Rite. You can customize in full in a later life, but there are various Christian church situation mechanics that you only access when you're not heretical.
Once you make your custom rite, use your holy legend's Evangelize the Realm decision. It will convert 50% of the counties that didn't convert when you changed religion and rite. This will make Japan a majority of your new faith, minimizing the number of county conversions remaining. You will likely be the fourth largest branch of Christendom at this point.
Once you make your custom rite, convert each and every vassal count or house head to your new rite. Many who followed to Carolingian Christianity won't make the jump to your new rite. Converting between rites requires piety, which is the main reason you wanted Carolingian Christianity as your starting rite. This manual conversion wave is to avoid counter-conversion later that would destroy development, especially if your new rite becomes heretical and your prior Carolingian brothers now try to convert their counties to their now-oppositional faith.
Be warned that this conversion phase will cause a lot of social upheaval and stress-quakes that kill large numbers of people. Characters who convert to religions they're unfamiliar with will take a full level of stress and become sinners with no piety. That stress will especially kill the elderly, causing more stress, causing more casualties.
This is a good thing, actually, at least when it kills most of your rival families. This can be useful as a point to deliberately phase out enemies and other families from the realm, but a lot of your own family will die to.
Have a quick family-only funeral to mitigate a lot of stress, and get your family into positive piety ASAP. It will take a generation for newly born Christians to get respectable piety levels, so be sure to host funerals on cooldown as able, with a guest list screened for family, in order to give your dynasty the biggest edge.
Yamato Christianity Combo
My custom rite for Ritsuryo Japan- what I called Yamato Christianity- is To Work Is To Pray, Love Thy Neighbor, and Scholasticism. It is focused on stacking development and tech development bonuses for long-term growth.
To Work is to Pray gives everyone inspections, costed in piety rather than gold, and a 20% development buff. The personal tenet lets you take the develop capital decision, and gives the gardener trait, which itself makes one better at the court gardener position that Japan has unlocked for capital development growth. At the realm level the bonuses of this tenet scale the more single-county counts you have to take that decision, and no one has more counts than Ritsuryo government. This is a huge long-term gain towards raising Japanese development for the tech game, since base development growth is rarer and more impactful than development % modifiers.
Love Thy Neighbor makes Hospice buildings give more plague resistance and travel safety, the former of which is good for reducing development loss in plagues. It's also a good building to replace those bothersome vassal fortifications with to defang house revolts. As a personal tenet it's worth a lot less, but it can offer +15 vassalization acceptance, which can go a long way to diplo-vassalizing nearby islands if you convert their children to your faith and culture first.
Love Thy Neighbor's signature perk is that counties that reach 60+ opinion get the modifier of the same name that also offers +0.2 base development, and a 20% development buff, and a similar control buff, and more popular opinion beside. Given that every county capital receives +20 opinion (thanks Ritsuryo), and you can add another +15 from religious doctrines, and even more from Christian breweries in temples, and even more with Japanese festivals, and Japan has almost unmatched popular opinion potential that can be brute forced onto the AI.
Scholasticism is a rite you'll only get via the Prophet Perk since you can't access a certain Irish monastery, but it has absurd gains. +10% culture fascination, before the Court Scholar position who can add another 2-10%, is already great. -50% university cost would be great too, except it's even better because going now gives the Scholasticism character modifier, which is four separate skill-per-level-of-devotion stat buffs for up to +5 diplomacy, martial, stewardship, and intrigue. As a personal tenet you don't even have to go to university, and will get a 10% lifestyle XP, and a boat load of book inspirations, and the ability to make book artifacts- including the legendary one you got from the holy legend- into holy artifacts for custom holy sites.
Put it all together, and you have an insane synergy of development and tech acceleration that synergizes beautifully with a country that generally won't be declaring declare external wars until the mid-900s (unless you force a law faction right before assuming the Kampaku position).
Key Doctrines
There are three core doctrines to this build, and everything else is a nice-to-have as the divergence budget allows.
Alms and Pacification for Clerical Function, and Encouraged Monasticism are core picks. You lose a 3% of gold income (from +30% for being Ritsuryo), but get +15 popular opinion towards Love They Neighbor.
You want a Pluralist religious attitude. This will vassalize infidels instead of clearing them from your land, which you really want to do against the meritocratic vassals of east Asia. Unlike Ritsuryo, Meritocratic Dukes have titular MAA, which Ritusryo influence can call forth. This more than makes up for the military power potential between Ritsuryo and Soryo governments. Losing access to evil vassal revocation isn't a big deal, as once you pay to convert tribals into castle holdings you can revoke Ritsuryo vassals without tyranny anyway.
For pilgrimage type, you want the Local Rites. This makes pilgrimages more expensive, which you can afford as the player, but gives access to short (less than one month) pseudo-pilgrimage where you can trade ~150 prestige (or gold) for equivalent piety, as opposed to the 4+ year pilgrimages to the middle east.
Everything else is a nice-to-have. Marriage type doesn't really matter- Japan has polygamy via culture. The key point is to ensure divorces are at least conditionally allowed. For crime, admin rulers can be revoked without tyranny anyway, so this is more about arrests for ransom or hooks if you criminalize male infidelity in particular.
Divergence Buffer
While you have a 65 divergence budget before heresy, glitches in the tenet-change process mean you realistically want an 11 point buffer in case an ecumenical council changes tenet or doctrines. This should buy you enough time to reform your rite / call your own ecumenical council to re-align your divergence before another, heresy-making, change. Of that 54 budget, 25 should be reserved for assuming no tenets are mainlined or banned (Scholasticism is Unknown for 15, the others are known for 5 each), while 11 are for the core tenets.
That leaves 18 points to work with for further customization. Your three most significant options are criminalizing male infidelity (5) if you don't expect to be fornicator yourself, de-criminalizing Witch into merely shunned (5) so that it's not a sin or crime for your future-heirs to be caught practicing, and/or allowing both male and female clergy (10). Allowing both clerical genders uniquely synergizes with Warrior Monks and Fragile Peace, as it allows your martially-trained women to not be exclusively defensive warriors. This is an excellent way to get good use out of martial or prowess daughters you none the less don't want to add to he family.
Post Reform Dilemma: To Diocese or Not To Diocese? (Don't)
Once you reform, you have an option to reach out to the Pope and Rome for a specific interaction that ignores diplomatic range: to create a new clerical region. This will set up an archpriest much as exists in Europe. You can even set up two archdiocese in Japan, one in your capital and one in the conquered north.
This has a few advantages. This is required to start a grand cathedral project, which offers local bonuses and is required for a custom holy site. You can even puppet both archbishops as Emperor, one as your realm priest puppet and the other as a clerical ruler puppet.
Don't create a clerical region too early.
When you reform your custom rite without an archdiocese, you are the head of rite. It does not change without your consent. But when the archdiocese is established, the bishop can take it, and then the bishop can change it. And the bishop will change your awesome tenet combo, and you can't change it back, even- or especially- if you end up heretical and your own new faith.
This is because Ecumenical Councils to manually change mainline doctrine require a quorum of 5 archbishops. But Japan cannot form 5 clerical regions in the early game. There are only 4 kingdoms on the main island, and you need external expansion from the law behind the early medieval tech to be able to wage war overseas for new eligible areas.
Japan can't even form clerical regions for the 4 it does have. I'm unclear on the exact requirements, but believe you can't form a new clerical region in your de-jure realm if your capital is already in a clerical region. So the Japanese emperor can form one for the capital, and the one in the non-de jure northern kingdom, but can't form the middle or southern island clerical regions without changing capitals... and even that would be only 3 in the first lifetime.
Additionally, even if you do form a clerical region, most of your vassals won't be able to take advantage of the biggest potential gain of the conversion in the first generation anyway- clerically-founded duchy titles. (I think they still work in Japan- I didn't get a message saying it was impossible due to government reasons.) The title request requires a certain level of devotion, which dropping to sinner for the foolish conversion means they'll lack, meaning it'll be a generation or so until normalized Christians can afford to request.
As a result, it'll be better to wait until you unlock external wars, and holy wars, with Royal Prerogative in the early medieval era. You can get 3 kingdoms in as few as 3 holy wars, a Kingdom in Korea and two island-kingdoms to your south. Then you can request a new clerical region, before requesting your archbishop immediately de jure drift them into your empire.
This would give you 5-7 Kingdoms to request clerical regions for, enough to have a reliable quorum for local ecumenical councils. Once you have a quorum, you can at least revert control of your faith's doctrines if they are a heresy.
But you'd rather avoid that. If you can stay aligned to the Roman Rite past the great schism, as heresies take Europe there's a non-trivial chance that you can become the mainline rite by size alone, at which point your ecumenical councils can regularly reassert the 'proper' Christianity after any European councils you can't affect.
Post-Reform Developments and Expansion
Over time, maintaining the faith is a matter of regular construction. Your doctrines reduces the heresy outbreak risk, but your size makes fervor hard, eventually impossible, to keep up naturally.
You have two main ways to keep fervor high: build temples, or use puppetry abroad to force holy wars against your faith. The former can leverage your great Ritsuryo economy to generate more Love Thy Neighbor opinion and development via breweries. The later can leverage religious hostility and block confederations for easy defensive wars where the loser gets vassal-swarmed and pays you gold and fervor for the privilege.
As Ritsuryo, your government can permanently subdue the Soryo by raising crown authority. Level 3 crown authority lets Ritsuryo counts become Soryo, but not declare internal wars. Level 4 prevents them from converting in the first place. There is a reason to tolerate Soryo counts emerging at level 3, which will be covered later.
Level 3 crown authority also lets you pass the imperial pacification law for external wars, including your first holy wars. The most impactful Kingdom holy war will be into Korea.
The Korean peninsula starts as 3 Kingdoms, two being absorbed by the King of Silla. If you want to maximize future ecumenical seats and smaller Kingdom-tier meritocratic vassals, you want to preserve those kingdoms. The easiest way to do this is to puppet the Sillan king and then have him release those de jure duchies. However, if you leave/help Silla absorb it all, you can take the entire Empire-tier peninsula of 13 duchies in a single Kingdom-scale war.
The alternative is Balhe, north of Korea. It is a medium-scale meritocratic realm of 5 duchies. It also is at risk of being bullied into extinction by the steppe nomads, so early conquest preserves that shield. The issue is getting into range for holy wars, which have to be within 2 sea tiles. You can use the naval county cassus belli for a non-holy war stepping stone into range- there's a specific Silla county-island that starts as a tributary that makes a good target. Balhe tends to have non-dejure territory outside its borders, meaning you can usurp the Kingdom title itself after a holy war.
The two most important duchy holy wars are to the south, to Ruukuu and then Taiwan. Both are small single-duchy tribal kingdoms, but drastically expand your diplomatic range into inland China and offer easy ecumenical seats. Taiwan in particular opens up two holy war avenues: coastal China, or the Philippines.
Coastal China is the most developed / quickly profitable expansion zone, albeit only during a hegemonic shattering. Being meritocratic, holy war vassalization can give titular MAA access. It has the highest concentration of unique buildings, and the most small-kingdoms. The entire de jure empire of jing yang is nothing but 2-or-3 duchy kingdoms that make efficient meritocratic vassal states. The main risks of China are that if/when the hegemony reforms, it will have de jure claims on the coastal empires. However, you can use christian ecclesiastical interactions to immediately de jure drift territory into the Japanese empire, which is outside the de jure or consolidation casus belli. A good schemer who abducts rulers or arranges alliances can easily paralyze the early chinese crackup to prevent a military snowball from reconsolidating China, especially if you puppet Chinese kings into foolish wars against you or yours.
The Philippines is a longer-term investment with eventually great Ritsuryo potential. The plentiful hills and jungles offer tons of manor houses and rice paddies with the rice cultivator tradition if/when your Risturyo vassals convert or hybridize the local cultures. The local Wanua tribals are mostly harmless, and can even provide buffs from trade missions. Small-domain Mandala can be neutered by naval county conquering their capital complexes, or puppeted into suicidal holy wars.
The final expansion route of note is the tribal islands of the north. The Kingdom of Aynumosir may be dominated by a powerful tribal defensive confederation that brings a lot of tribal MAA potential. One way to mitigate confederations is to introduce a religious hostility to them for a diplomatic unraveling. The first to introduce an heir to organized faiths has an advantage.
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Spreading The Faith (Through Kidnapping Children and Chinese Chaos)
Okay, so you have a church. How do you spread it, especially before external wars are unlocked?
The easiest route is the proselytizer personal tenet which lets you attempt to convert anyone whose county you're in while they are home. This costs piety but your manor-boosted stat scaling (and gifts to tribal chiefs) easily makes acceptance guaranteed. This is ideal against unreformed pagans in the north and south.
Alternatively, exploit espionage skill stacking. Use this for rampant kidnappings hook generations to get future rulers into your religious pipeline. Abduct and recruit adult heirs to your court where you can convert them and matrimonially marry them to your daughters. Keep them in your court as long as possible, so that they can benefit from your more frequent stress-relieving activities and event decisions before being new rulers abroad.
One of your more interesting opportunities is the inevitable 867 Chinese crackup. Due to how it starts, there are a lot of Kingdoms where the nigh-inevitable heir is 3 or more childless old man successions away from a title, and thus game for a matrimonial marriage without alliance considerations. Kidnap, dungeon, and torture is a rapid killer, but 'just' kidnap and dungeon doesn't run afoul of christian laws or spiritual fullfillment.
Not only does a lot of Japan-Chinese Kingdom alliances (or rulers in prison) keep the era of chaos from resolving with a new hegemon, but it also introduces the potential for Yamato-Chinese cultural fusions. This is important because two of the Chinese cultural traditions (Scholar Officials and Cultivation) enable characters to convert to Confucianism/Taoism, undoing your religious progress. A Yamato hybridization (probably) won't have that.
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Conclusion: Long-Term Endgame Challenges
What's left to say? That's enough I think. Too many thoughts.
