Not OP, but Signal is a pretty good choice in my experience. Giving a "general overview" for them is tough, but the best I can think of is "Telegram-looking, Telegram-experience, but using the Signal Protocol, which is considered **a** gold standard. Centralized servers off-shore (I think). And no bots."
Alternatively, I've heard good things about Matrix/Riot. Federated, very much like Mastodon. Personally I couldn't use it because the iOS and Android apps are just "not there."
Some information on both is available on Privacytools, including other alternatives. :D
https://www.privacytools.io/software/real-time-communication/
Junior NOC/SOC analyst | Interested in DFIR, and DevSecOps | Opinions my Own |