Only thing seems to consistently work is Twitter - and then you should REALLY be building public if you have no audience so that you can build one. Have you tried this? If not, and you've been building in stealth mode, you've missed out on an opportunity but you can still start now.

You are figuring out so much stuff in an area without expert - make a Twitter post every day about what new problem you solved and you will quickly find a following as you establish authority in this new field. So many people are learning from each other about this stuff, it doesn't take much to become an expert.

I've been going through all the Indie Hackers podcasts. Consider listening to Indie hackers #008 or #009, I think they specifically talk about bootstrapping an audience from nothing.

You may even find people are more interested in an educational product from you than your service, and pivot, for instance.