You can add the device you want to use PiHole with to your WireGuard VPN and set the DNS property for the Interface.
Then you need to add the PiHole machine to the VPN, if it is not already reachable (or you use your existing machine in the internal networking with masquerade)
Getting a better rack. My 60cm deep rack with a bunch of rack shelves and no cable management is not very pretty and moving servers around is pretty hard.
Hardwarewise I’m mostly fine with it, although I would use a platform with IPMI instead of AM4 for my hypervisor.