This sub is pretty open to self-promote your own project: https://www.reddit.com/r/SideProject/

But to you answer question:

I'd avoid self-promoting to a niche sub. You'd be in danger of getting banned immediately without any explanation. Just update your user profile with your project info and comment on each posts in the sub. Be helpful, provide value, and be fun. People will click on your profile for sure and DM you. But this method takes a long time to work and can be pretty draining.