This would make my sketches much better fully in favour of this option, also to keep the airbrush from building up even at low opacity. Like a uniform glaze that doesn’t start covering up everything if it’s at quarter opacity.

Agree.

I believe apple’s notes app does this somehow. It would be a nice inspiration for its simple and convincing results. Although we have a little build up on the opacity, it’s pretty much under control and it doesn’t get too dark...

Opacity would be ideal.