A note on your point about "tensions": such tensions would be invoked if there was a form of ducal independence. Of course, there are always tensions in a big realm (somebody doesn't like somebody else), but once a higher authority steps in and tells both parties to play nice, it mostly puts a stop to it. With ducal independence, you'd have a nice fight over such discussion.
Dukes can always play hell with a realm if they don't want to support the government. Pull your army back from the front lines. Refuse to provide gold for the ruler's pet projects. Reassign all your knights to your own army and make yourself the Marshal, then keep them home to do police work and defend your duchy. Have all your lords sell their food to you, and not to your neighbors. All of this, of course, depends on your ability to influence your lords and knights. But then, if you can't do that, then you're not a powerful duke after all. You're just a figurehead.
