Thanks all, that’s great feedback.
I originally looked at adding cache, but then thought that even if i did, it still left read speeds at a single spindle once the data was off-cache. If I moved to striped, then I could reuse my disks and get better performance without needing to buy SSDs!
The other thing I looked at was Ubuntu 19 + Cockpit GUI.
This gave me Docker & KVM as well as the (beta) Cockpit ZFS GUI manager.
Would have preferred Centos 8, but they have moved to Podman instead of Docker and getting the right combination of ZFS / Cockpit / Podman all working together was problematic.
Only looked at FreeNAS as it was the quickest to deploy but ProxMox sounds like it could be worth a look as well.
Looks like I have some testing to do over the Christmas break!
Happy Holidays!