I'm ready to replace overpowered pfSense rig (my previous gaming rig). It too much power and heats up the room. Now that summer is approaching, I don't need this extra heater and I want to replace with something lower power and quiet.
Once I do testing on the Up² as a router, I'll let you know how it performs. Base model is half the price of the embedded solution you listed. It should work well. It does have a fan, but you can't hear it. It uses such a low-power SoC that if you really need it to be fanless, just stack a larger heatsink on it. power consumption under load, I figure to be around 5-10W if you connect no peripherals.
One alternative is finding an Atom-based machine or something based on an AMD E-series CPU. Those will run you a lot less than $150, especially considering you have a lot of the other parts you need. Slightly higher power consumption for sure, but far below 70W.
from looking at the old video Tek Syndicate did back in the day, they introduced very briefly an ALIX box which runs off an AMD Geode processor, so I imagine if that can push gigabit, an E-350 should be able to as well, but I don't actually know.
Also, yeah, those are the atom boards. They're pretty dated, so that's why I lean towards the AMD boards, but I haven't used one. my last router used a 1.86GHz Core 2 Quad, so...