Probably won't apply to you but for rackmount I have a Gigabyte GA-IMB410TN + Pentium Gold G6405, in a 1U Logic Case SC-1280, with a quad-port 2.5 GbE Realtek PCIe NIC (r8125) to supplement the motherboard's dual 1 GbE Intel NICs (igb & e1000e).
The motherboard can run off of 12-19V DC input via 5.5mm barrel jack (e.g. any old unused high-power laptop charger), or 12V via a 4-pin CPU EPS power connector (they are commoned together). Since the case comes with a 250W 1U Flex ATX power supply, I'm doing the latter.
This lets me take the PSU's 12V rail out via the barrel jack (edit: with 18 AWG cable, I know) to power the modem and a couple of TP-Link 2.5 GbE network switches which all run on 12V, saving 3 wall warts. Both switches are velcroed to the top lid with 20cm patch cords for 2.5 GbE ports 1 and 4. I have 5 LANs at home, so the 6x wired ethernet ports this gives me are perfect.
OpenWRT runs pretty great on amd64 and I've been running the v24.10 branch (pre-release) for several months on it without issue.