Techniques for painting with a soft brush over already painted areas?

For simple shading, I like to use the “Protected Colors” and the “Repaint Brush” options to paint by hand using the brush. Unfortunately, this doesn’t work with soft brushes (it then fills the whole contour of the stroke without respecting transparencies on the edges, for some reason). Is there a way to accomplish this quick way of shading by hand, restricting it to already painted areas of a single color?

The only thing I’ve found as an alternative have been using gradients, but that doesn’t give me hand control with the pencil like the brush does, or maybe using masks ( ), but I’d rather save myself the hassle of setting up nodes for each individual case… there must be an easier way!

If someone more experiences could share some ideas, I’d really appreciate it. This is my only major grip with this software since I suscribed, and has given me enough headaches to consider checking other software alternatives…