Ubuntu as router, with routed traffic going through ExpressVPN?

So as said, I have ubuntu server. Cuz I'm in China and bc of censorship and stuff, I kindda want it to also behave as a router, with a VPN always connected.

In my house, I currently have 4 routers for various purposes. What I want the ubuntu server to do is that it puts all the traffic into the expressvpn app, while the traffic that itself is hosting, such as minecraft servers and other stuff, to pass through the normal chinese internet. Is that possible? haha. Kind of like split tunneling for devices connected to the server.

For the server, itself have its own ethernet port and I also have my own usb ethernet card (it works with ubuntu). Does that mean I should do something to route only the traffic from the USB ethernet card through the VPN?

Heres a diagram for clarity:You can ignore the modem and the two routers before the server.

While I do know that it is easy with an openvpn config, or I can just set up VPN on another one of the routers, the thing is, openvpn does not work in China. Only the lightway protocol in the app can actually bypass the censorship :(

Any help will be gladly appreciated. Thanks!