Food for thought: a brief treatise on the useful super duke, and how to properly employ him.

I, your fellow Machiavellian Emperor, micromanager of the realm and steadfast adherent to the principles of vassal education and general pragmatism herby propose an already known but little used (to my knowledge) policy of super-dukedoms.

For best use, acquire a titular duchy, although a titular kingdom can work, as can small duchies or small kingdoms. Ideally (in vanilla) placed duchies for this are the duchy of Cyprus, any of the 2 county Irish duchies, or any vassalized holy order (I haven't done this in a while, I assume its still doable?).

Be warned, this vassal will be very powerful, so you should proactively be training their heir(s), this is a pet dynasty of yours, and can even be a "cadet" branch of your own dynasty if you'd like (be careful with this, it's possible for this dynasty to politically marry if you extensively land branches of your dynasty).

So, down to the nitty gritty:

Grant them 1 county in every 3 county or greater duchy, and leave the rest to their de jure issues. This duke will be ultra powerful, extremely loyal (+100 at all times) because you're granting them counties. They'll never really acquire more land (because they're content) and their forces can be raised anywhere in your realm, to put down rebellions, to act as frontline defenders, or to act as the spearhead for an attack.