You're still making the assumption in this comment: why does my 2nd (cloned) database need a separate postgres instance? One postgres server can host multiple databases.
My point is that for the use case of offering a Postgres service with CoW branching as a key feature, you can't really escape some form of separation of storage and compute.
Btw, don't really want to talk too much about it yet, but our proprietary storage engine (Xatastor) is basically ZFS exposed over NVMe-OF. We'll announce it in a couple of weeks, and we'll have a detailed technical blog post then on pros/cons.