Don’t know your car, and I assume there’s probably a reason you don’t, but what about just opening the windows?

It’s been a long time since I actually used subnets, but IIRC you will need a physical interface for each network on the router regardless.

So let’s say you set up your /24 network into 2x /25’s, you will need an interface for the .0 network, and another for the .128 network

If you just have an interface for the switch, and another for the WAN connection, I don’t think subnetting will work for what you’re trying to do