The system has a resonant frequency that depends on the damping, mass of the motor, and brush friction. Those set a hard limit on the maximum possible speed.

PDI will get you close to that speed, but there's always going to be a trade-off between speed and accuracy.

From the test, you're still quite a way out from perfect repeatability. (I prefer the automated version, because the smoother strokes look slightly better.)

I suspect you'll find that PDI with a thicker medium like oils/acrylics will be harder because the brush friction will be less consistent. And in fact real paintings are often created with a range of brush sizes and perhaps a range of palette knives - so building a robot to handle all of that isn't going to be trivial.