have you tried CasaOs or Zima board? It's their premise your own micro cloud

They're missing the key feature I'm looking for: Decentralized backup to the same devices owned by people I choose. That's the "someone else's computer" part of what I want in a "cloud."

I can already easily run such things on my home computer. It's having remote (encrypted) backups and redundancy if my own system goes down that I'm looking for.