[jan-jaap]

Here's a more comprehensive look at how I'm set up, it's not very complicated and it works really great.

I'm sure you had fun constructing that but I wouldn't recommend it to random people. If you don't trust your ISP modem/firewall, I would recommend something like pfSense or OPNsense. I used to run pfSense on a PC Engines APU2C4 which is a passively cooled platform using only 5W I think. servethehome.com has reviewed countless mini PCs that will do fine as well.

Both pfSense and OPNsense come with a web UI, documentation and are actively maintained.

The Cisco 3600 series were last sold 20 years ago, and have been unsupported for 15 years. They apparently use up to 140W of power. I'm sure it was a fine choice in it's day, but just because OP wanted a firewall for his IRIX system(s) does not mean the firewall has to be of similar vintage as the IRIX system...