Have you tried influencers? Personally, I would go for finding some small influencers that can generate content for me, that I can, after that, use for my ADs. They don't have to be big ones. Even people with 1000-5000 followers would be good. You don't even have to pay them, as a lot of them are using APPs like yours and would be more than glad to review yours.
This can be beneficial because you are, at first, generating organic reach, so you don't have to worry about the ADs competition and high expenses.
Aside from that, find a few relevant hashtags and do your best to get trending in them. If you generate an initial amount of organic followers, things will be way easier after that.