I just caught the Pi undervolting again (how dare it!).
In all seriousness, the problem seems to be simply that the Pi is trying to drive the electronics in both my Creality printers. I haven't tried tried the TronXY because reasons.
So I wonder if anyone here has tried making up a USB cable with the +ve 5 V supply severed? It's absolutely necessary to keep the common (0 V) because otherwise the communications won't work and ... the result there is probably obvious.
However, doing this should prevent the Pi from powering the electronics when the printer is turned off. I've noticed this when closing down for the night, that while the printer's PSU is off, the displays still suggest the printer is powered. And yes, I've tried printing while the power switch was in the off position. I've printed myself a nice dunce's cap for that one. Twice.
Any thoughts? I'm going to see if I can find some solderable mini and micro USB male headers - I've found a full-size type A but as you might imagine it needs some fairly tricky soldering to do correctly. Surgically cutting the +ve on an existing cable should be possible with care though.