You might be interested in Peergos which lets you easily live mirror to another instance and everything is E2EE.

I guess this is kind of like what I mean.

What I really want, though, is literally just for there to be an Umbrel "Backups" app that lets me choose as a backup location one (or more) friend's Umbrel(s).

Redundancy is the main thing all these Docker-wrapper systems are missing for general use.