How to tunnel all the traffic for one interface through another interface

I have two working interfaces on my ubuntu machine:

• tun0 - OpenVPN server with the internal address of: 10.8.0.0/24
• wg0 - WireGuard peer interface with the internal address of: 10.7.0.0/24 which is connected to an external WireGuard server with a successful handshake.

And I'm trying to tunnel all the requests for tun0 through wg0. So when I connect to the OpenVPN tunnel, it would be as if I'm connected to the WireGuard server.

And this is how my iptables backup looks like before making any changes:

*filter
-A INPUT -p tcp -m tcp --dport 443 -j ACCEPT
-A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -s 10.8.0.0/24 -j ACCEPT
COMMIT
*nat
-A POSTROUTING -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to-source MACHINE_IP_ADDRESS
COMMIT

So by looking at this I figured if I replaced the MACHINE_IP_ADDRESS with the internal IP of WireGuard connection it might work:

-A POSTROUTING -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to-source 10.7.0.0

Which did not work. I also tried:

-A POSTROUTING -s 10.8.0.0/24 ! -d 10.8.0.0/24 -o wg0

And in both cases, the OpenVPN connection was established but fully timed out.

Is there a way I can achieve this without too much configuration?

That sounds to me like a "reverse proxy" behavior, you could use Caddy or nginx for this, I think it should be fairly easy, something like:

caddy reverse-proxy --from 10.8.0.0:2080 --to 10.7.0.0:9000

With iptables, I'm not so sure but I found:

https://serverfault.com/questions/1025857/iptables-as-a-reverse-proxy

Which seems very similar to your solutions, but it seems they map one port at a time, did you try to map a specific port?