Success! I never was able to measure any shorts, but I decided to try reflowing it again just for the heck of it, and it worked. Connectors are a bit melted now, but that’s ok.
I had given up on my closed-loop CNC project due to the wires running between the RP2350 brain and the motor driver step pins picking up noise and causing spurious step interrupts, but stepstick can plug into my original GRBL board which should hopefully have better grounding.
I have to use HybridStepperMotor with 3 channels for now due to my incorrect choice of timer channels (can either use two complementary pairs or three non-complementary channels, whereas StepperMotor needs four non-complementary). I could write some code for the dual complementary stepper scheme I had in mind, but since I only spent a few bucks on bare boards I’ll probably just order the new version I made with the proper arrangement. And the old ones are still perfectly usable for 3-phase brushless motors.
EDIT: I forgot I never actually uploaded the images for the new version:
