Another thing to keep in mind is creating extra de jure kingdoms causes law confusion. Say you are the King of Ireland and have Low Crown Authority and you create a second kingdom. If you want to move to medium authority, not only will you have to pass a vote in the de jure of Ireland, but you will also have to in the second kingdom. It makes it especially annoying when you're trying to keep vassals from fighting one another but end up with multiple de jure areas that essentially become the wild west of vassal relations.
I had that problem when i played as the Rus and created the Lithuanian kingdom. Not only did I have every character with the lithuanian culture plotting for it, but when i moved up to Medium in the Rus, Lithuania stayed at low.
In my opinion it's better to keep one de jure King title and let your de jure drift take care of the rest.

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