I covered one quarter with good old black Sharpie, a lacquer crayon, and well-aged black acrylic wall paint:
Applying a sanding block removed the rubble + scribbles and brought the surface down to the engraved patterns:
The lacquer crayon doesn’t seem to adhere well to styrene:
A closer look shows I probably sanded off too much of the surface, perhaps above some grit below the sheet, because those lines almost vanish:
The crayon may adhere better to deeper lines. These are obviously too shallow and the pigment seems to come off in chunks:
The acrylic trim paint filled its patterns, despite having turned into a gummy mass during decades on the shelf:
The Sharpie ink, being basically a thin liquid, completely filled its patterns and (apparently) soaked into the rough side walls. The lines seem to be 0.1 mm wide at 225 g downforce:
They’re less uniform at 250 g:
A 300 g downforce produces (somewhat) more uniform 0.15 mm wide lines and slightly distorted characters:
I have no way to measure the actual engraving depth. If the 60° diamond tool had a perfect point, which it definitely doesn’t, then a 0.15 mm wide trench would be 0.13 mm deep. I’ve obviously sanded off some of the surface, so those lines could be, at most, 0.1 mm deep.
All in all, the engraving came out better than I expected!