That's basically how SmartOS does it, as long as you're doing SMB or Samba. And boy, is it ephemeral: the entire OS is loaded into RAM from a USB stick (usually) on boot, and everything gets loaded from the zpool, as OC said, and if you need anything, you put it in a zone. Even VM hypervisor goes in a zone, because it's essentially free and gives you nice clean separation (and quotas, reservations, etc.).

Though NFS can only be shared from the global zone, so if you need NFS you don't get to play with zones.

> the whole thing should be marketed more like "Proxmox with batteries included".

It's not though. Even if you make all the "NAS software" part of the jails (or the half-baked Kubernetes in TrueNAS SCALE), it's not even close to Proxmox when it comes to managing VMs, containers/jails, firewall, networking, etc.