I have setup my NAS with the IP address 10.0.0.10 (which is not in the scope of my DHCP Server - 10.0.0.100 to 10.0.0.199) and have confirmed the netmask, gateway and DNS server are all correct - but hours after everything is up and running, I find my NAS “forgets” it has a Static IP address set and…

Yep, DHCP deamon is definitely still running. Have you tried just resetting the NAS since you changed to static?