I looked into the Netwatch feature https://wiki.mikrotik.com/wiki/Manual:Tools/Netwatch and it seems to do the trick! Netwatch will watch a host with pings, and run a script if the host is up or down.
/tool netwatch
add comment=“pi-hole failover” down-script=“/ip dhcp-server network set 0 dns-server=""” host=192.168.88.15
up-script=“/ip dhcp-server network set 0 dns-server=192.168.88.15”
my pi-hole is 192.168.88.15
if up: /ip dhcp-server network set 0 dns-server=192.168.88.15
if down, blank it out (or set it to 1.1.1.1 or 192.168.88.1): /ip dhcp-server network set 0 dns-server=""
my dhcp network settings
