You can do this right now with an CM4 attached to the seeed studios dual ethernet carrier:
https://www.seeedstudio.com/Rapberry-Pi-CM4-Dual-GbE-Carrier...
For $120ish I'm not sure you could beat the value prop of one of these boards with a used laptop. Supply is certainly an issue and was even before COVID. Waiting until 2022 sucks, but I'd argue they are by far the best overall option for a DIY whitebox router I've personally seen.
Yes, its absolute overkill for any sort of hobbyist use, and its not cheap. But it certainly has potential to be a networking powerhouse.
I'm not sure what's the heat is like if you do gigabit NAT with them though :)
https://en.t-firefly.com/product/industry/rocrk3568pc.html
RK3568 has 2xGbe built-in, and the board has additionally a Wifi 6 chip (broadcom), seems like pretty ideal router platform to me. There are even some hints that it might have half-decent mainline support, but who knows really though..