Raithmir

I don't see why you'd want separate clusters, just stick em all in one.

2-3 second hand small form factor PC's running Proxmox, cheap 2 bay Synology NAS for backups.

2 vdevs with 10 x 10TB drives in RAID-Z2 would give you around 150TB useable.

So you could do that with one vdev and 10 x 20TB disks, but you wouldn't have as good performance and 20TB disks take a long time to resilver if you have to replace one.