Hi john, I'm fairly experienced in networking as well however you probably hav more knowledge however I'll give my 2 pence :)
the outbount NAT rules are used from the top down. Have you tried placing tHe NAT rule at the top? Also
make sure the destination is set to the DMZ sub net
also, would you not expect to see requests coming from your WAN IP? like when you surf the net, external servers will see your WAN IP…
You probably can so some fancy footwork to get the DMZ servers to see the DMZ if interface but I imagine it required adding rules manually
cheers