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I have an ECS mini pc with a Core i3 and dual nic that I use as a router at home. For normal internet usage it stays well within that 5-10 watt range. Only slightly more than a raspberry pi but with much, much more performance.

For 10-20Watts you can use a slightly dated office-desktop (thin-client) that will take a pcie nic and usually has more than enough processing power.

A step further would be a laptop that can come by with 5-10W and still run circles around typical arm-based off the shelf routers. thou the second nic will probably connect via usb3, you get a real keyboard and screen for troubleshooting.

I use a NUC clone with a J4125 Celeron as a Debian file server and it hovers just under 3W. These tend to have only one Ethernet port so you will need to add additional ports on USB for use as a router.