There is finally a decent alternative to reddit, after one has been needed for years? And it’s open source federated software?? AND I can sign up on an instance that supports free sharing of media?!
I have been waiting a long time for reddit to fuck up badly enough that something else could fill the vacuum they created and do it right. Awesome.
People from reddit started joining only 2 weeks ago, it will take some time for the platform to mature for sure. And regarding niche communities, your best bet is to make them yourself and start posting on them to attract people that are interested in the stuff.