Set the AllowedIPs wireguard setting (and/or the route, if you can set that separately) to one larger than your home network (i.e., if your home network is 192.168.1.0/24, use 192.168.0.0/23). Then, block wireguard packets from the internal network on your router. Then the tunnel will always be running; it just won't be used when you're at home because there's a more specific route