I did build my own router, two actually, but opted into buying one of the Deciso appliances with OPNSense pre-installed for the main WAN connection from my house to the ISP upstream.

The biggest reason I did that is that I wanted four ports, not two, and I could not find a mini-ITX motherboard or any other commodity form factor with four ports built-in. Once you have to add an expansion card, you need a case quite a bit larger than what you're used to for a home network appliance.

That's an underfilled niche, small form factor single boards with more than two high-speed NICs, but presumably it's underfilled for a reason, which is there aren't many individual consumers, as opposed to third-party packagers like Deciso who sell appliances to enterprises, who want these. The other consideration is server-grade mini-ITX boards that have even two onboard NICs have a whole lot of other features not needed in a networking appliance that end up consuming more power than is ideal. If you could offer such boards, with or without a case, with or without a pre-installed operating system, I'd definitely be interested, but I'm one guy. Don't build a business on scratching the itch of notoriously stingy homelab enthusiasts. We're the kind of people who will gladly pay $400 a week on groceries and then scoff at a $10 a year software subscription as something unconscionable.