Are there any other small hardware devices that can run off a USB port? Or easily flashable old smartphones?
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.