Have you tried using the Brush Controls' Brush Calibration panel?
If not, it was introduced in Painter 12 and allows us to set the sliders as we would using Brush Tracking but for the individual brush variant. After the sliders are set to your satisfaction, you can save the brush variant with a unique name not already used by Painter and have the settings just right whenever you use that variant, or until the way you're painting changes enough to warrant resetting the sliders and saving the brush variant again.
Another thing you may want to do is to use the Painter 11 brush variants in Painter 12. They're installed with Painter 12 so that should be easy enough to try.
Other than that, the only thing I can suggest right now is that you make sure your Wacom Tablet Properties are set up right to allow the brush variant to do what you expect.
Hope this helps!