Not sure what's causing this exactly, i don't even know where to look tbh.
I've checked for squareness and it looks fine,
The belt seems to move a little vertically over the course of its motion.
The attached video shows an inverted V motion after homing, which doesn't complete.
This movement works at accels close to 5k and under. Sometimes for >5k. Repeatability is 50-50. Sometimes it works fine.
When i try printing something at those accels i have layer shifts towards the front-right. Each layer prints fine but the next layer considerabl offsets over the previous.