Sweet, that command is amazing, should do the job nicely. I was thinking either some kind of SD hot swap card with a switch might exist (SD-card and switch is no longer a searchable term, thanks Nintendo)
connectors are these (ATX 24 pin), not DB 25, I think I'd loose my wits trying to solder one of them. Did see some with a screw top but once again budget and as you say current capacity, plus I've never liked DB connections always found them unsatisfying somehow. The Ideal ones I found but once again budget were EDAC/Varelco 8016/516.
Sweet. I'm going to start with the 3d printer flavour before getting into the CNC, since I think that's the one that will be trickier.
the second part - using DB25, make sure you use high quality DB25 as PRC DB25 can't handle more than 200-300mA, decent quality DB25 can handle 1A on the pin while you can find high end DB25 that can handle 5+A per pin. Signals are not the problem but motors, heaters ... digikey is your friend here, mouser is ok too