[spongechameleon]

I think something like Ricochet (if it were still actively maintained) could be a good solution.

https://github.com/ricochet-im/ricochet

Every user is their own Tor onion service, so you get E2E encryption and no centralized servers. The whole thing hinges on the security of Tor itself which is probably a safe enough bet.

[blepyzdiec]

Briar https://briarproject.org/ is similar, plus, AFAIK, it's using E2E on top of Tor service. However, looking at massively caught Tor services, I believe, to keep it truly secure, you have to rotate your service address, aka reinstall the app, losing all your history of communication.