Along similar lines, I'd say Snikket. I feel XMPP often has quite a bad reputation based on the user experience from 10 years ago, but it's come such a long way and projects like Snikket make it very easy to get started.

Defintely agree here. Glad that Signal finally allowed users to stop connecting their phone numbers to their accounts but man, I wish there was something else.

https://github.com/privacyguides/privacyguides.org/blob/HEAD/docs/real-time-communication.md or https://www.privacytools.io/privacy-messaging might have information useful to you, or if they don't but you find better solutions for your needs please let me know!

You could check out Wire, Session, and Threema etc.

Signal and Simplex are also pretty good.

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