I played around with ZeroTier over night and was quite impressed with it.
My LAN IP range is 192.168.2.x
My brothers LAN IP range is 192.168.1.x
I installed ZeroTier on his Plex server (where Wireguard is also installed). and added him to the network. I installed ZeroTier on my Raspberry Pi (where Wireguard is installed) and added that to the network.
Then I made his assigned ZeroTier IP static, 10.241.x.1
And then I added a new route: 192.168.1.0/24 via 10.241.x.1
And sure enough now I can access anything on his LAN from my LAN (to do this with Wireguard, I would typically have to tell Wireguard to send ALL traffic over the VPN, meaning while connected I would lose access to my own network resources)
EDIT: There is a wee bit more work involved in making that work, but a chunk of it was already done when I set up Wireguard. There are some firewall rules that need to be added to the “exit” node.