Diagonal engraving will give you the best quality in my opinion. I think the issue is the feed rate is too high. Try the settings I suggested and we’ll go from there. A “simpler” pic won’t make a difference. The software reads the image one pixel at a time and converts color values to grayscale (0-255). The complexity of the image doesn’t affect the speed at all. The bottleneck is the serial comms between the host software and the Mega and the Marlin planner and command buffer need to be able to keep up with the selected feed rate. Like I said before, I never laser etch faster than 1500mm/min. I need to strep out so I won’t be able to test your gcode until tomorrow for comparison but I think you’re on the right track.