After spending all day messing with it, I can say you don’t want to try running UGS with a Raspberry PI 4B and a SuperGerbil board.
Running a job works fine. Enabling the spindle and jogging does not. Jogging will fail in the same way if the spindle is off, it just takes longer. Doesn’t matter if anything is connected to the pwm, motor driver, etc. - just being on in software is enough to “make it worse”.
Something in this setup is causing GRBL to reset while jogging. Using the exact same cables, board, software, UGS version, etc. that I’ve been using for many months on a Kangaroo PC.
I’m going to blame the USB on the RPI4 for now. It’s the only think that makes any sense at all, and it’s still pretty weird given how it manifests.