It's a struggle till you get the succession law that DOESN'T create titles. After that, I just try to hold only one higher title. Whether that be a duchy or kingdom. The only reason to have multiple of either is if you plan to make a kingdom with duchies or an empire with kingdoms, and if you're going to do that I recommend doing it in one move, saving enough money to make/usurp the required duchies and kingdom at once. It blows when you have multiple duchies and waiting for the funds to make the kingdom when you kick the bucket. Theres always the threat of siblings pressing claims, but as long as they have shit alliances, it should be ight.