I only have a GPU because my CPU doesn’t have any graphics. I don’t use the graphics anyway, but I need it to boot. So I put our crappiest spare GPU in (GTX 750 Ti) and call it good.
I wouldn’t bother. If you end up needing it, it’ll take like 15 min to get it installed and drivers set up and everything. No need to bother until you actually need it.
Here are two somewhat reasonable routers that support 10G (via 2 SFP+ ports):
Both have max power draw under 50W, though I don’t know what they’d actually draw (would depend on how much traffic and whatnot).
And here’s a switch with 2 SFP+ ports with max draw of 11W: https://mikrotik.com/product/css610_8g_2s_in.