I improved the Z coupling, made a much better pen holder, and discovered trochoidal milling, which, in addition to making actual milling easier, draws cool pictures if used with a pen.
The pen holder was actually somewhat complex. The pen I wanted to use was a Bic 4-color, and its barrel has a very slight taper to it, so I had to model it fairly precisely and it took two prints to get the dimensions right. Now that it’s done, though, I can get really nice, repeatable drawings. I’m thinking of developing an algorithm to convert images into combinations of the pen’s four colors and emit gcode to approximate color printing. It will be ugly due to color theory (there’s a reason printers use cyan/magenta/yellow/black), but it might look neat.