Whole platforms to be honest. Since you’re already out of corpo land, lets keep you out. Matrix is the best I can think of since it combines a lot of positives. I dont know other decentralized group chats except maybe IRC. You can download apps for matrix on matrix.org, apps for irc you will find on your favorite search engine or if youre a linux user, the software store. Mobile phones and tablets of course have them too.

Then you check for your topics. Mental health, specific neurological things, interests, etc. Go from there.