Earlier quoted context omitted.
The only p2p messenger I've used that hasn't suffered too badly from "blank canvas" is Manyverse. It's technically not a dedicated messenger (more like a p2p encrypted Facebook equivalent, with a p2p FBMessenger equivalent built-in as a side feature), but that aspect just makes it a lot easier to find people to chat with. It's unfortunately not quite as stable/performant as Briar though (yet).
I did have a bit of a play with Manyverse (and with SSB itself in the past) and it looks interesting. The lack of syncing between desktop and mobile SSB accounts is a bit of a pain which I think will hinder adoption until they come up with a solution to that. I do worry that a lot of these newer messengers and networks are operating on a memory of what social networking was like in 2010. Quite a few at least handle p…
Yeah that's something they're actively trying to figure out, but it's tricky given their "Signal-/WhatsApp-esque" single-device architecture.
I think some of the work Matrix are doing on federated key exchange could be of some help though; my general understanding is that multidevice, single-account encrypted communication is hard.