Matrix and XMPP are excellent options worth considering for your use case. XMPP is more mature but modern clients implementing all the right RFCs are a bit scarce, Matrix clients are coming along relatively quickly but there's the occasional quirk.

Both have optional support for features like federation and bridging other chat clients as well, though the results are often not exactly perfect.

Personally, I'm quite happy using Matrix+Element/Cinny/Fluffychat to gather several chat services into one place. Experiments with native group chats have also gone down as of a year or so ago when encryption finally got most of its quirks worked out.

You should check out Element.

> This would be interesting if E2EE was officially supported

The reality is people claim to need E2EE but it's not worth paying extra for it.

It's kind of like how people nod when you ask them if they hate companies like FB/GOOG stealing their personal information and then you offer them a dollar to give your their DOB, Address and SSN and they happily take the dollar and share all of that info.

… and those really serious about E2EE are using their custom stack anyways.