I think this is a great discovery/idea, in general, also the result looks beautiful, thanks a lot for sharing it
It might be interesting to delicately apply painter's tape to the keycap before painting to produce large but firm characters
However the hand writing obviously has it's own charm, looks beautiful, so the painter's tape idea is unnecessary
How's the texture / rigidity?
I think a stronger/permanent marker could be better, if it exists. don't like how it turns out? draw a new keycap, yet the drawing should, in my opinion, be permanent and not re-workable
With these kind of mods, if it starts pealing off after some time, it converts joy into nightmare