It should work just fine. the proxmox just sets up a bridge to the virtual interface when you install it and after that, VMs you deploy will appear in to everything else on the network as if connected through a switch.

However, watch out about it your routing tables. If the pihole isn’t on the same network (regardless of if it is virtualized or not), you have to tell other computers on the network how to reach that other network, by making changes to their routing table, or the one in your router (that is literally its job, after all).