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.