That sounds kinda similar to the problem Matrix solved with cross-signing, how when you login to a new device and verify it with one of your already logged-in d

That sounds kinda similar to the problem Matrix solved with cross-signing, how when you login to a new device and verify it with one of your already logged-in devices, it can request your old message keys E2EE so you get all your history. Maybe a similar thing could be built on top of it?