A little more than a month of weekly quick sketch sessions plus practice at home has yielded the top sketch, a half-hour one, and bottom, a more typical 5-minute sketch. I'm using a Procreate brush that changes width with speed so I can get a nice fine line if I use quick strokes, but the line still has some variation. The top left swatches are my progression from light line to darker if time permits.
The top sketch shows why this method is so seductive. There's so much potential to go quickly from a sketch to a finished piece. But it's not easy, and for me still a real struggle.