Almost literally any cheap SBC will work, but multiple ethernet would be an advantage. I don't know of any that have a decent wireless capability built in, and although it's probably possible, I haven't seen anyone use a PCIe or NVMe slot that higher-end boards have for wifi either. If you're going to use a USB adapter for wifi, be prepared to take a serious hit to usability with multiple devices and range.
Hardkernel has great boards, high quality, comparable prices.
Pine64 just announced a RISC-V board with dual ethernet, but I don't know when it will be released.
You can probably find decent OpenWrt/pfsense/etc-capable routers on ebay or Craigslist though - if you find one that can run an open firmware, it will probably work better for firewall purposes than a homebrew SBC kludge.