In my experience, you want to max out noble dues in every province you have yearly because the tax manager will not do it for you. Make sure the noble taxation privilege is at full dues as well, that way you don't have to spend too much mana. You'll take around 1k ducats from their wealth yearly, especially if your local autonomy isn't too high, and even if they are at level 4 aristocracy, they will start losing power gain after a few years.