One qualm I have with these external TMC2130s is that they have a pretty finicky bootup procedure. They have to receive motor power first, then IO power, otherwise damage can supposedly occur. So I have the motor power broken out to a separate power supply which is switched on before the main 3d printer supply. Even then, powering on the 3d printer incurs several seconds of loud grrring and twitching from the motors connected to the TMCs before things settle down.

I was confident this was caused by one of the IO lines being noisy before the printer's microcontroller got to the point of initializing it, so I set out with an oscilloscope to find out what each line looked like during power up:

It doesn't look like these pins are causing the problem to me, no crazy noise or serial signals (Some of the gpio were also configured to be i2c display outputs). Unfortunately the documentation on TMC stepsticks seems a little hairbrained, so I guess my next stop is to take a look at the upstream datasheet from Trinamic and see if there's anything I can figure out from it.