I am not going to lie but I joined Zulip for some project and I really enjoyed it.

Stoat (formerly Revolt) is great for single server (ie. no federation between multiple servers) and matrix/xmpp are good if you want the latter (ie federation)

Good to see some more options though but all the services I shared are open source. Not sure if this is open source.

I don't like to use closed source services (usually) because then I am still trusting trust but I am gonna be honest that Closed source services make more money at times so there is a trade-off for some businesses.