Thanks for the update.
We do have a 3W EmBlaser (aka LaserBlade) here with standard reinforced belts in a CoreXY configuration, but it does not have to move allot of weight like our Shapeoko does. It has two puny steppers on it too. We had major problems with it achieving any quality laser photo/image engravings when we first received it after there Kickstarter campaign ended. We had to make some mechanical modifications to it which they implemented the changes after our recommendations.viewtopic.php?f=13&t=35
We also changed to these grbl settings which increased the EmBlaser's performance.
$0=30 (step pulse, usec)
$1=255 (step idle delay, msec)
$11=0.005 (junction deviation, mm)
$120=10000.000 (x accel, mm/sec^2)
$121=10000.000 (y accel, mm/sec^2)
I suspect some of the canvas effect is caused by the .1mm Pixel Resolution setting your using. The burn lines has to be overlapping, especially when engraving at a 45D angle which makes the burn lines even closer together than using the Horizontal & Vertical angles. The laser's burn lines should be laying down next to each other without any gaps, or overlapping. Not only does the PR set the step over, it also sets the step ahead, so those very short .1mm incremental moves may be causing a slight stutter, especially when using grbl 1.1f. But, the amount of stuttering will depend on your X&Y accel settings.
Have you tried increasing the PR setting and have you flashed the MultiMode grbl 1.0c firmware to your board like I suggested? If your still using 1.1f, I don't recommend activating the Dynamic Laser Mode with the M4. It may be contributing to the canvas effect too, but I could be just speculating. If you get darker burn lines around the edges from instantly reversing directions, use the Extended Edge option instead.
What gcode streamer are you using? Our PicSender was developed side by side with the EmBlaser to stream the very large, fast changing raster gcode files our image to gcode programs create. Not all grbl gcode streamers are created equal!