And another #FreeBSD host installed successfully. 😁 After I retired the Proxmox VE host at my parent's house, all services ran on the FreeBSD NAS, but now the secondary DNS jails run on their own thin client once more.
VNET with bridging didn't work, so I used Netgraph instead.

The realtek re0 driver received all of the ethernet frames addressed to the VNET jails, but the bridge0 interface (I tried the new VLAN aware bridging) didn't receive them from their parent interface.

Luckily, I can just create VLAN interfaces on re0 and use Netgraph together with Bastille to create netgrap bridges and netgraph eiface interfaces, which do not require driver support, as they directly hook into the network card's send/receive buffer.