The Deep grooves could be caused by the stack-up of ‘rounding error’ related to the holding power of the stepper motor and the Line interval. Some of the split-steps or ‘microsteps’ in between the full steps don’t hold very well.
If I knew how many belt tooth ‘pockets’ are on the pulley on the drive motor I could assume the motor has a 1.8 degree step and 2mm belt pitch and make a good guess as to which way to adjust the Line interval.
May I ask what you have selected for DPI or Line Interval?
The background ‘grain’ looks like the curtains artifact that belt driven devices make.