I learned early on to never fix someone’s computer, or let anyone know you were doing anything with computers because as soon as something breaks it’s both you’re fault and your problem.
Made the mistake of building my own router last year and I haven’t heard the end of it. Any outage is immediately my fault. Any slowdown is my fault. Broken links, banned accounts, internet bullies, all my fault.
No! A pi makes a terrible router, you need something with a bit more power. Flash openwrt onto a cheap wireless router you already have and use it as a dumb access point for an opnsense router.
I built mine out of an old fanless pc meant for running digital signage (usually super cheap on eBay). Has like 6 HDMI ports and 2 nics. You can get a computer with more nic ports if you want more vlans/isolated networks. But for most home uses, a 2 port setup works well. One for communicating with your ISP and one for communicating with your dumb OpenWRT access point.