GIMP 2.9 may fail to start due to some MyPaint brushes failing to "deserialize", whatever that means.
In order to "solve" that you don't need to get rid permanently of such brushes, which may still work fine on MyPaint itself.
Instead, move/rename the .mypaint user folder (perhaps ~/.config/mypaint to some?) to mypaint-temp or whatever, run gimp 2.9, it will start. Then go to preferences, folders, and change the mypaint-brushes folder settings, but