The key to get successful grayscale engraving results is to use as much of the PWM range as possible. Theoretically, your only using 1/3 of your Laser's power range with a Min Z-0.03 and max -Z0.23. That is only a possible 1/3 of the shade range you can burn.
Do you have a way to monitor the PWM voltage? I use an Analog Modulation laser driver and monitor the output voltage with a 0-5v analog voltage meter. One of these. https://www.amazon.com/uxcell-Rectangle ... tage+meter
If you have one, wire it into the analog voltage side on the PicConvert board so you know what voltage range you are getting. Generate a small gcode file with the full range, or slightly less to see if it returns back to your Z axis zero place at the end of the file each time. I would experiment with the step & dir pulse setting to find out what works best for your machine not to lose any steps. If you have not already, set the X,Y & Z at the same velocity & accels. The accels should be set at the highest value possible to where none of them lose any steps.