Pi-Hole on RPi, different ip on eth0 and wlan0-- Issues?

Set the DHCP range to less than the pool of available IP's on the LAN, then use an IP outside of the DHCP range. No client will be using that. Even if a client were to use the "unused" IP, unless the client has a DNS server listening on port 53, no DNS traffic will be answered.

IP range 1-254. DHCP set from 100-200. Use IP 225 in the router, for example.

Using an unused IP is not functionally different than using 0.0.0.0 in the router settings. Neither will get a DNS response.

Another option (and perhaps a better solution) is to disable DHCP on the router (or limit it to a single IP, the IP of the Pi, if the router does not allow disabling DHCP), and use Pi-hole as the DHCP server on the network.