Personally, I'm using a Haswell era Dell SFF PC as a router, running OpnSense. It's fairly low power (I think about 15W idle? been a while), has ample performance, and a PCIe slot to stuff a proper NIC in. Put a 4590T in it to get the power down, and replaced the PSU with a custom board to run off a 12V supply directly instead of mains.
Can easily put 32G of RAM in, multiple SSDs, wifi if one really must put it in the wrong place..