I’m not saying that the problem is the Y axis but, don’t be certain that it can’t be the problem because a slight skewing of the Y axis will angle the #3 mirror and cause the beam to change position at the build surface. Besides things which can physically change the position of the mirrors, you have to envision problems which can also change the orientation of the mirrors and affect the final beam position.
One of your tests( 2 rows of 10 alternating direction lines ) shows misalignment exaggerated in the center lines. If you do this test again with 2 rows of 20 lines and then repeat the 2 rows of 10 lines beneath the 2x20 lines does it show the same offsetting lines in the same set of vertical lines?
Can you print out a 1cm x 1cm grid on a full A4 size piece of paper and then with 0,0 in the back left corner(use beam pulse/fire button) line up the outside corner of the grid with 0,0 or 5,5 or 10,10(which ever allows fullest area coverage). Clear and then set your origin at this point and pin that grid corner down. On the MOVE tab set Distance=10mm and Speed=30mm/s. Now move( Y-only ) programatically(LightBurn’s move command) and align the right rear corner of the grid with the laser beam rotating the paper on the pin.
Tape the paper in place and now start programmatically moving your laser around the grid and make sure the beam is lining up with the grid from left to right and back to front.
I would start at a few spots(6) across the rear(x,0) line then move to the center(x,maxY/2) and finally front(x,maxY).
- Does the laser beam hit the grid intersections consistently?
- Moving the laser backwards(right to left/front to rear) to the previous spots, does the laser hit the exact same burn holes made when starting from 0,0 going left to right/rear to front?