

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
You could simply continue to use the older versions - with Linux, you’re not forced to update if you don’t want to. Since it’s open source, you could even continue modifying it freely. I think the people behind Linux are being a bit overkill in supporting older hardware in the most recent releases.