That works and it's a great writeup, but it's a lot of manual work. Just get OPNsense (great if you have the horsepower) or OpenWRT (for a smaller machine). You'll learn a lot doing from scratch, but initial setup and maintenance will be much easier with a purpose-built distro.

I've been running various homebrew routers for close to 20 years now; OPNsense is fantastic. Bonus, run it as a VM on your Proxmox host and eliminate a few wires!