PvE has a button to restart Pi.hole (or anything) and I usually use that - but I have tried reboot before with the same effect (though I should re-test this as I think that could have been back on Pi.hole #2.)
I'll see if I can work out how to do that - I'll admit, once we get into editing files, I get lost quickly as I have no idea where to put additions or even how to ensure they're saved properly (I barely worked out recently how to open a line via nano - and that has to be given to me ahead of time, no searching for this noob!)
I'm not familiar with Proxmox, but as you want Pi-hole's DHCP service, you probably should verify that your container allows for the required capabilities, specifically CAP_NET_ADMIN - see also Note on Capabilities.
I'll research this tomorrow - thank you for the lead! Will report back shortly. Hopefully that also gives some time for any other ideas to flow in too! Thank you and all!