@nico buddy, I used to work for Cisco. I worked at an ISP for years. This is just basic shit that I've done countless times except instead of a Wireguard tunnel it was another point to point tunnel like a GIF or a GRE tunnel, sometimes with IPSEC layered on top.

It's. Just. Routing.

If the traffic for a subnet is routed to a router, you can route that same traffic out another interface over another tunnel. You just need the route in your table so it knows where the next hop is, and it will work as long as you don't exhaust the TTL. It's not rocket science. This is how the entire internet works. How do you think you can reach Google if your ISP doesn't own Google's IPs? They have a route saying "send traffic for 8.8.8.8 out this interface, someone on the other end knows where it goes after that"