

“centred around” is a subjective projection rather than statement of a fact in cases where gamete production genuinely does not occur. For this person, her gonads never developed into either testes or ovaries, so by this definition she would be of neither sex. I’m OK with that, but it does undermine your point about the strict binary.
My question to you is why does this matter, in the context of accessing bathrooms and changing rooms? Do you think inspecting reproductive anatomy is a proportionate measure?
More broadly speaking, what is the point of recording the ‘biological sex’ of a person who, through transition, has changed their physiology and endocrine profile to that associated with the opposite, and no longer has their natal reproductive anatomy? Who would this benefit?

Having additional virtual network interfaces on VMs is completely normal, ens18 does indeed sound like the right one you should be looking at.
Seconding the other commenter who mentioned the possibility of a second DHCP server.
Is your Proxmox host wired via ethernet to the pfsense? Or are there WiFi APs in the mix anywhere