I've lost track of your devices... I think you're saying your FW has three interfaces: two LAN ports, one for inside and one for dmz (so each is a separate physical LAN), and WAN attached to the ISP box (which is in bridge mode). If so, are you making additional VLANs on inside and/or dmz or are you just working with the one LAN on each?

Is the "(new) wifi/8-port ethernet router" routing or just acting as a switch for your inside LAN, attached to the FW/port1?

Or am I completely confused?

Regardless, I'd personally prefer to keep all the DHCP on one device for simplicity. I hate having to keep track of the subtleties of two different user interfaces.