Seconding the XMPP reccomendation- I run Prosody, which has been rock solid and stays up even when other services I run go down. File sharing, E2EE, reactions, voice and video calls- everything you’d expect from a modern messenger. Cheogram or Conversations are great Android clients, and Monal is the iOS client I’d reccomend.

The tradeoff is that there’s a Guixy way to do things that might not be obvious from the upstream docs, so could be harder to get e.g. a service up and running initially, but for me the reproducibility makes it a good tradeoff.

Been daily-driving Guix on 3 different machines for about 3 years. Love the declarative configs and rollbacks, and being able to easy share configuration with Scheme snippets between machines. I know that once I get something working on one, I have it permanently working for all my machines.