If you set a secondary DNS server, your clients will just use it to get ads, negating the effect of your pihole.

Unless, of course, the second DNS server is a second pi-hole ;p

When the raspberry pi (running pi hole) is down for any reason

You can have 2 DNS servers provided over DHCP assignment, if this is really an issue for you (ideally it shouldnt) you should be running 2x PiHoles for failover protection, that way you can take 1 offline and all devices will auto swap to the second one for DNS.

This will also horizontally scale your DNS querying, as devices will just 50/50 flip flop between the two while both are online and it will sort of auto-load balance.

Largely speaking thats your best bet though, just 2x PiHoles for failover protection.

For DHCP just use one of the pi-holes DHCP, or your routers, or whatever, they all largely perform the same but I personally find the pi-hole’s DHCP settings to be easiest to work with (you can even super fast modify them by SSHing in and using nano/vim to text edit the config file, so instead of using a UI you can just copy-paste all the entries in quickly)

If you need to take the DHCP pi-hole offline, you can just flip DHCP on for the second one, however as long as new devices arent connecting you don’t actually need the DHCP server online once everyone has an IP assigned until their registration time is up (24 hours by default) so as long as you get the pi-hole back online quickly, you’re network typically wont even have a problem.