I once wanted to build one like this, but for console devkits. That situation was and went, but I did end up wanting to have the same for all windows machines at work and home (Unity dev, so workstations, test runners even servers are desktop windows machines).

My solution was:

-BIOS: auto-power on when power is connected

-Smart power plugs on important computers

-AnyDesk for software on all machines, with unattended password set (connectable even at login screen, less annoying for licenses than Teamviewer)

-(Optional) my own WoL sending / restarting / RAT program for all the machines too

-(Optional) Parsec also installed, to allow remote desktop good enough latency/control wise for actually gaming remotely (which I often did!)

This way, I will never be stuck in a situation that I can't access a machine that I'm away from, unless it's one of those without a smartplug and has somehow hard crashed. Costs less than 20€ per machine. I've NEVER needed to access the bios or similar remotely.

I've recently played around with a lot of SBC's and have a spare capture card and cheaper sbc's that could work as OTG keyboards. Might end up building a crappy version of one of these for fun