* strongly encrypted
* open source
* distributed, or at least not requiring a hosted service
Assuming that cryptographic math claims of widely used algorithms are true, as a basic matter of professional competence, these security actors have incentives to compromise implementations. It's tanks against crossbows. At best.
If you are asking, it's probably more like tanks against a reader of books on crossbows. Being clever enough to thwart a lone wolf in a hoodie doesn't scale to actors who can compromise hosted services in ways that the service stays up and the service doesn't discuss the compromise.
When push comes to shove those professionals apply wrenches. All ordinary security is theater. But ordinarily theater is enough and anyway that's all ordinary people can afford. Good luck.
https://matrix.org/blog/2022/09/28/upgrade-now-to-address-en... has our take on it, fwiw.
Fwiw, I believe Matrix (or possibly an XMPP+OMEMO setup) is the only current solution which fits the requirements of the OP.