If you've ever built a computer, just put together something lowend. MicroATX is usually least cost and easy to work with.

Something like https://pcpartpicker.com/list/3MpGmk

Some people don't care for Realtek nics, you can spend more to get Intel if you want. You could put more ram, or storage, or whatever if you're going to do more than routing. Ryzen might be nicer, but they don't have a cheap CPU with integrated graphics right now, and it's not worth paying $100 more to get one for a router, IMHO. If you can find a motherboard with IPMI for cheap, that'd be nice, but I can't, so I just use a spare monitor and keyboard when I need to.

There's small x86 router boards around, and they're cool, but if anything goes wrong, your options are limited. If anything in this fails, you can get the part replaced at your local computer store, even if that's BestBuy.

I typically use whatever I have laying around. Right now I've gone atypical since I currently have a 10 gbps symmetric connection and my leftover hardware isn't quite fast enough to route it.

Get a mini x86 device as a wired router on which you can run any number of OSes (openwrt, debian, *sense etc.) and a separate AP (ruckus unleashed devices work well).