Asklemmy

Clear vote for Matrix.

Easily works with basically every system from iOS, Android, Windows, Linux,Mac and there are Web-Clients as well. And everyone can find their own client of choice.

You can use one of the huge federated instances like matrix.org or set one up yourself.(Which is much easier than setting up an Activity pub instance,btw.)

There are rooms/channels that can be open or semi-open just like in Telegram, and bots are absolutely not problem (Not Like in signal). Registration is not bound to a phone number like it was in signal for a long time (but is no longer) and (unlike still needed in signal) a phone does not need to be used for sign up&transfer between devices works seamlessly.

If you selfhost or use beeper you can even bridge to different services (signal,WhatsApp, telegram) etc. so you only use one app for everything.

I run my own XMPP server. https://joinjabber.org/ has some introductory information Snickket is a all in one service you can host or your can pay them to host https://snikket.org/start/

XMPP servers and clients are a lot lighter weight than matrix, though I think matrix has more fancy features.

They're both self hostable and are federated to share the costs of running the network.

With signal there's a non-profit that needs some pretty steam funding to stay in business. (Though they're popular so maybe they can get it) https://www.pcmag.com/news/want-to-run-an-encrypted-chat-app-youll-need-50-million-signal-says