Oh and the messy borders when Karl dies is because the AI doesn't know how to hand out titles properly. Like he gives the Duke of Aquatine control of Flanders or transfers the count of Bourbon to the Duke of Brabant. Or he hands out multiple duke titles to one character. And then when he dies, France, Aquatine, Burgundy, Mid France, and even Germany looks super messy. With the king of France controlling Saxony and Aquatine ruling Holland.
Dunno how much it would help with map gore, but Carolingian vassals should be mostly viceroyal:
By the early 9th century, the efficient administration of Charlemagne's Empire was ensured by high-level civil servants, carrying the, then non-hereditary, titles of counts (in charge of a County), marquis (in charge of a March), dukes (military commanders), etc. During the course of the 9th and 10th centuries, continually threatened by Viking invasions, France became a very decentralised state: the nobility's titles and lands became hereditary, and the authority of the king became more religious than secular and thus was less effective and constantly challenged by powerful noblemen. Thus was established feudalism in France. Over time, some of the king's vassals would grow so powerful that they often posed a threat to the king.