After this, I’ve started a classical forum (flarum)

NodeBB recently added fediverse support

Mastodon for personal use: not so much as replies don’t federate with main tweet.
Mastodon for small group: Maybe.
Lemmy: If you want to host a CDN-like node then yes, but not everyone likes forum style discussion.
I didn’t use Frendrica Sharkey or Pixelfed enough to form my own opinion.

I can’t deny that