Pi 4 with PiHole and Access Point as failover

Hello,
I have a Raspberry Pi 4 and want to set up PiHole to ad-block my whole home network but also have an SSID that has no filtering. My router has a private SSID and guest SSID sharing the same network. My router serves as DHCP server and tells clients that it is both gateway and DNS resolver. I can tell the router to hand out PiHole’s IP as DNS, so I should be able to ad-block ok. However, I want a third SSID named Emergency that uses the same gateway, but my ISP’s DNS. Then if PiHole stops working my wife can switch to the Emergency SSID without knowing any technical details if I am not home.

What I would like to do is connect my Pi4 to my router via Ethernet and use the Pi 4’s WiFi as an Access Point for the “Emergency” SSID. I think I can run two DHCP servers on different machines on the same network as long as their IP ranges don’t overlap. I also think I would need hostapd to run the AP on the Pi. If I don’t use PiHole as a DHCP server, can I manually configure DHCP on the Pi to report the router as gateway and ISP DNS as router and have it only respond to requests that come in from the wlan0 interface?

I am new to this level of networking detail, so maybe what I’m asking for is silly. But is it possible? Would it work? Would PiHole updates reconfigure the DHCP server running on the Pi?

Thanks

Pihole is very stable and you will create a kind of Frankenstein that way. If the Raspberry crashes your wife still have nothing. The WiFi on the Raspberry is also not the strongest one and better use the one of the router.

If Pihole crashes you can still reach your router by using its IP address and change the DHCP. Then the clients have to reconnect or reboot to get the new address of the DNS server.