The Laser Everything Video is excellent in details of how to achieve the best possible dpi/lpi for any laser and/or material. Might help you get a better image if you could dial in the dpi/lpi…
Understand it’s the heat from the black paper that is causing the glass to shatter. Usually referred to as the indirect method of engraving.
To get more shattered glass you need to put more heat on it. If your laser is burning the black, it will probably go through and heat up what’s behind that…
That does a couple of things, lowers the possible resolution because the size of the heat area increases. It changes the speed/power values as now you’re lasing a different material wanting the same effect… The paper will react differently than the mdf…
Since you are already a half mm out of focus, this will just aggravate the resolution and heat issues…
Go to ACE or something and pick up some thin glass, lase, properly focused through he thin piece → black paper → heating the glass tile. It will be on the other side, so make the proper adjustments.
You can at least get proper focus… and see if that helps your depth. The simple truth is it will be difficult to get much depth out of this setup… doesn’t mean it isn’t possible…
The ntm and TiO2 have numerous links on this topic…
I wish I could do what @Bulldog does… 
this kind of quality has eluded me so far…
This is on a co2, I belive.
This is his fiber …
Good luck
