Well just found out that Vmware are deprecating Vfrc in the next 6.7 update as they believe that caching has more effect at the SAN than the host.
So I will be making adjustments to my NAS, I will relocate the Optane 900P to the spare PCIE x4 slot and carve it up to cache both SSD and HDD pools.
I will relocate the Freenas boot from the USB drive to the Optane 800P NVME drive.
I will purchase another 2 x 1TB SSD's and replace the 2 x 250GB SSD's, this will lead to a mirrored pool of 2 Vdev's (4 x 1TB SSD).
I think I will use the 250GB SSD's as ESXi boot drives in my hosts, then nothing relies on USB drives anymore.