Thanks a lot!
I did try the digital painting brushes because I also had in mind that, especially the Digital Round, did exactly that, but none of them interact with the paint underneath at all. Maybe that is a bug then?
I did try your recommended settings on a different brush though, incase someone wants the same thing, these are the settings I'm using for a brush now:
General:
Flow 50%
Wetness:
Blend Transparency: Add
Mix Rate: 3
Blur Radius: Flat
Flat Hotspot: 80%
Wetness: 70%
Constant Paint: 6% (and this has the biggest influence on the outcome xD )
Autoclean and Auto Load off
Edit:
Resetting brushes helped as well. Weird! But problem = solved, thanks again, fjg!