Ive seen some people have had success with USB, but I have had endless issues with it. It just doesn't read fast enough, even 3.0 seems to be slow or have too many disconnections that disrupt the machine. I don't even read post files direct from the USB stick these days, I always make sure to copy the files to the PC and read from there, as anything to do with USB results in missing steps. I actually know this issue from my experience in tuning cars as well, you do get fairly regular drop outs and connection errors when connecting to the ECU via USB. Ethernet would really be the ultimate, but none of these machines are using it.

Mach4 isn't really much more modern for what these machines are being used for. If you were doing 4 or 5 axis machining then yeh Mach 4 would be the go, but otherwise Mach 3 is still just as good.

And yeh, you need to setup custom power settings regardless of what PC you are using. Ive always just used the one PC so I can't really comment otherwise, but I know when I was getting into it I read a lot about people using Windows 10 and finding there were just some setting that weren't configurable that were causing issues. Some people came up with work arounds, others ended up just ditching it and getting an old PC.

I wouldn't say these are rules, I'm sure you can use other equipment and make it work, but I think it is just so much simpler with the older tech and less to go wrong. Too many fancy functions and too many processes/things running in the background on newer computers. Every little thing that interferes just causes inaccuracy, and when your job ends up drifting 1 or 2mm during the operation it's hair pulling at best and damn right infuriating and expensive at worst.