I read a thread on r/Procreate a while back where someone said the same thing, that most of the time lost on repeated work comes from redoing the setup, not the drawing itself. That stuck with me because it matches what you're saying about the fur. It takes a bit of guts to stop and build the tool when you could just keep grinding out pieces. Tweaking the spacing and taper is the part nobody talks about though, and it sounds like that weekend was really more like a weekend plus some rough first tries. My own keeper is a simple tapered ink brush I made for line weight, and now every clean sketch I do starts with it.

Gotta disagree that it was fast because you were extra stuck. Explaining code out loud forces your brain to slow down and actually process each step instead of skipping ahead, which is why you caught the bad variable so quick. The duck isn't magic, it just makes you talk through the details your head usually skips.