Firstly, you will need console access to the FreeNAS box, Promox HV box (with the VM running) and the VM. You will also need UI access to FreeNAS and potentially Proxmox.
You will need to enter in your own pool paths and names.
In SSH session on NAS:
Put a hold on the snapshot you want. This will ensure that it isn't accidentally deleted.
sudo zfs hold ...
From my experience, Windows guests usually munch too much CPU when a virtual hardware driver is set to something less than ideal.
The settings I usually make are:
Storage should always be VirtIO-SCSI. You can just change this setting for an existing VM if the VirtIO-SCSI driver is already installed in the guest, but installing new guests requires you also ...