And, nice modeling!

When it really counts, we use the Copic Multiliner SP pens, which go down to an absurdly small “.03 mm” size. The numbers are not really comparable between different manufacturers. The black Copic Multiliner SP pens in the different widths are excellent (*except* for drying time), but the colored versions are not as vivid as Sharpies nor much thinner. You can get replacement nibs for (in all the different widths) for this series, so there’s some hope of switching in thinner nibs on the color pens, or switching in faster-drying ink. 

A roller-ball pen can also potentially work. Some people have had good luck with a regular ball point, but that requires extra downwards pressure, which can shorten the life of the pen-lift motor. A fountain pen might also work well.

And, not what you were asking for, but the finest of our kistka tips is *really* fine as well, and can make some pretty spectacularly thin lines.  We’ve been meaning to test these with just fountain pen ink, rather than the usual hot wax.