If you are experiencing issues with a Pi-hole and Apple iMac verify that the built-in firewall is not blocking Pi-hole. I had recently upgraded my Pi-hole server to an RPI 4 and everything worked fine except my wife's new iMac. DHCP services from Pihole worked on everything but her iMac. My temporary workaround was to manually assign an IP address and point the DNS to my home router.
The problem was eventually found to be the built-in firewall in Mac OS. After disabling it, pings worked fine and DHCP / DNS worked perfectly. As a permanent fix, I ended up leaving the firewall disabled and installed Little Snitch which is a better security solution and doesn't have any issues with Pi-hole.