With the 6W nubm44, I laser engrave photos at 5080mm/min on Birch Ply, or Poplar. At a 2000mm/min it will burn too dark. It looks like your testing on MDF. It will burn easier/darker than natural wood. You may need to back off the max setting or driver max amps (if adjustable) and use a B&W dithered image instead of trying to laser engrave grayscale.
I recommend you use a 3" (76.2mm) focal height. Focusing info can be found on Page 22 in our Laser Setup Instructions.https://tinyurl.com/yb6xc7gr