I use Gajim on Windows and Linux, Dino when I need to call, and Monal on Mac. XMPP is older and less trendy at the moment, and it’s a large, partially fragmented ecosystem- that might hold back mainstream adoption. But it’s the horse I’d bet on long-term.
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 recommend.
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.
I use deSEC, a non-profit based in Berlin. Has been an excellent experience, and very easy to use for DNS-01.