Just do static routes or BGP over Wireguard. Simpler, scalable, less error prone.
But doesn't provide layer 2 between networks. Think of devices that are hardcoded to communicate with broadcast or multicast with TTL of 1, you either need some active reflector, and cope with any perculiarities of the device, or you simply extend a single vlan between two routers (using vxlan or another solution)
I sometimes need to extend a system like this from one site to another. One is a calrec system (an audio mixer, I think it's the control traffic that needs to be sent), and I don't have enough access or time to see if I could build some kind of transparent proxy -- it won't work with multicast routing.
I do however have enough time to create a layer2 network between two nics. I tend to use mikrotiks for that, create an eoip tunnel (GRE with proprietary addons to cope with fragmentation) between the two endpoints and pop the interface in a bridge with a physical port, and move on.
