First off, let me clarify that I am a male dom who has had female subs(including but not limited to finsubs). I also frequently work with content creators, some of whom create NSFW content, so I have seen both sides to some degree and have a fairly unique perspective that I hope can be appreciated here.
My subs came from me spending a lot of time looking into and understanding what subs who like orgasm denial are into, and using that to create written content that was relevant and effective. I tend to be someehat good with words, and that helped too. I don't think I've ever messaged a sub myself (obviously). They all just DM after I make a post. I do have people randomly find me through comments or old posts fairly often though. The biggest issue I run into is that the majority of them just want to get off and then disengage after a few days. Of the handful that stick around, maybe half eventually get to a point where they've shown an interest in findom in some way. I've only had a handful who actually pay, but the ones who do stick around for a long time.
What I'd suggest looking at if you're having trouble finding a sub:
The subs who pay are the ones who stick around. Rather than wasting time on a sub the first time they message, wait a couple hours before responding, and don't do anything domme-y until the next day. My best experiences as a dom have been slow-burns where there's a lot of not being free at the same time to talk (at first), and nothing happens until days later.
Getting a sub to go from 0 to 1 is the hard part, so rather than requiring a $20+ tribute the first time, just ask for whatever the minimum the app will let them send you is ($1.00 for example). If they won't send a single dollar at the start, they will probably never be willing to send larger amounts. Also, a people who are good with money tend to struggle to justify paying even if it's a kink they have, but getting them from $0 to $1 literally can get them over that mental block.
The subs I've had who have paid the most are emotionally invested in me. I've found that having personal conversations (where I can be myself, not just a dom) and asking semi-personal questions that require them to open up just a little creates a lot of connection in a fairly short period of time. I also find that talking about things I want without overtly asking for them to pay for it, is a good way of keeping them motivated and thinking of what they're wanting to pay for in the future.
The subs who pay are the ones who feel safe. Setting spending limits is usually the best idea for this. It's also good to set expectations regarding what you are going to do, and also have a real genuine conversation with them now and again.
While some subs are willing to pay you for nothing in return, the majority of the money seems to be in the moderately impure side of findom (as opposed to pure findom where they pay and you do nothing in return) where you're giving something back in return for the payments. For example, if he buys you clothes, send him a picture of you wearing the clothes. If he pays for your pedicure, send him pictures of your toenails. If he pays for you to cum, at least describe how good it was. I've had a number of subs who were into that last one specifically and find it to be a fairly healthy dynamic.
The subs that pay the most tend to be into the most niche kinks, so specifying what kinks you're into can help. I would suggest either putting something in your bio, or having an about me post pinned to your profile. The about me page is strongly advised if you're actively looking for subs.
Quality subs do tend to prefer accounts that are older, have pictures, an about me post, other posts, etc. Also, PLEASE comment on things seriously rather than just "you slay queen" "Looking good ❤️" etc. If a sub can see you engaging with people in comments once in a while, it shows that you're online and will see the message. It also allows them to see how you communicate with people and get a sense of if they want to be your sub.
Finally, while I'm sure some subs do genuinely just want to be drained and looked down on, in my experience there are a lot who want to actually know you to some degree.
And just as a reminder, this is based on my experience with female subs and more indirect experience male subs through nsfw content creators.
I'm sure many of you have your own experiences, and I'd love to hear how your experience has differed from mine.