I run multiple pinholes using keepalived. Then I only use one DNS in my DHCP server. Second pihole will seemlessly take over if the first one goes down whilst using the original DNS address.
Work quite well. I had to learn the hard way that only using a single pihole was just asking for my partner to be mad when it didn't work / when I was doing server maintenance. Now I have multiple and they can all seemlessly take over if any my server nodes are down
Keepalived