>But I'm not aware of a messenger that provides a good UX for that

WhatsApp is the largest messaging app in the world (3B MAU) and does E2EE by default across private chats and groups.

Signal's current UX seems pretty good to me, and it's not even that easy to lose the account

> open source

I'd just stress that at least for the Android client it's open source with some degree of quoting: the real git history is not shared (each release typically consists of a single huge commit), and the source is often published weeks after a release of the official client.

The server is also not open source, but that's not very relevant for security.