Pleased to see that the Department of Digging Holes and their contingent of sub-contractors are still in business. The lane that I live in kept them busy for days. In my case they even implemented, "when you find you can't horizontally bore in one direction, then horizontally bore in the opposite direction".

E.g. Install OpenWrt (x86/amd64 version) on your PC.

That would be for a dedicated router. In my case, I would want to use
the same machine I use for data backups and some other long-duration,
low-priority functions.

Hmmm... An old desktop PC with 2 x Ethernets running OpenWrt would allow you to use a GUI for configuration and get a proof-of-concept device running and tested. You could then take a look at the linux networking that OpenWrt has implemented. ...then invoke this networking on your real router/firewall/backup machine.