Testing this, the main thing I feel I'm missing are a a way to set a specific value for the final density, that isn't dependent on zoom level. I think that would be a much more solid control, especially since it mirrors the real world quite well.

Other than that, a way to decide the final length would be nice. Though I understand it can be interpolated from the surroundings too, it would be nice to have explicit control sometimes.

If you want to look into those suggestions now, or you would prefer to save feedback on functionality for later, that's fine either way I think.

Testing this, the main thing I feel I'm missing are a a way to set a specific value for the final density, that isn't dependent on zoom level.

Well, the distance setting isn't dependent on zoom level. I had a density setting first, but found it much less convenient to use. When I picture a density in my head I think e.g. "I want a curve every 2 cm". Computing the corresponding density value from that isn't straight forward.

Other than that, a way to decide the final length would be nice. Though I understand it can be interpolated from the surroundings too, it would be nice to have explicit control sometimes.

Obviously somewhat important in general, but I wouldn't want to add more tool settings for that just yet.

For example, I'd like to type "1mm" and get a distribution of human hairs on a head that's somewhat accurate to real life.

In the test above the distance is basically doing what I want, I think.
I don't think the description for the distance property is totally accurate, since it's not just a distance from existing curve roots, but also curve roots newly added during the stroke.

Also, I guess the "Distance" property shouldn't display for brushes that don't use it.

For example, I'd like to type "1mm" and get a distribution of human hairs on a head that's somewhat accurate to real life.

Yeah, that is what it should do.