There's Matrix, which is federated (diff protocol than ActivityPub, though). It seems decentlly active but atm maybe one of those things where you need to already know people who use it to be able to find the good stuff, because I'm struggling to find much of anything I like (but I've also been dodging the larger groups because I'm nervous lmao)

There's also XMPP or Jabber as it used to be called, which is better for 1-on-1 type conversations, but group chats are also a thing there.

If it's just text, you could always go back to IRC https://en.wikipedia.org/wiki/Internet_Relay_Chat

I believe Matrix has all the same text and call features as discord, but I haven't tried it. Apparently there's also bridges you can run to mirror a discord into your Matrix instance.

#cyberfurz-official:cyberfurz.chat and #home:furs.chat are good spaces to join if you go with Matrix, maybe others could also chime in if they know any!

While it might not be fediverse based, I've been keeping an eye on an alternative cough clone cough called Revolt since they're aiming to have it be self-hostable.