@Bucking_Horn ok I tried the method with Conditional forwarding disabled and sorry you were right I did have Never forward reverse lookups for private IP ranges ticked
So my USG WAN is pointing to 1.1.1.1 for it's upstream DNS, my LAN networks are pointing to Pi-Hole and the Pi-Hole is using the USG router as it's upstream DNS.
The hostname doe appear to be working now but I get an odd insure message now for every connection:
What's happening here?