Just to point out: wealth tax used to be a thing. Bankers could set between 5-20% (or so?) on gold and bonds, past X limit. RTOs on rogue regions is no longer possible, it was quickly removed after how easy it was seen to be.
I'm also very reticent with all mechanics that inhibit growth. Growth comes from success, first and foremost. I see no reason to arbitrarily limit success. The solution to having both the "discourage or prevent realms where 100% of the nobles have titles" and the "encourage wars by allowing realms to grow as much as their military will allow it" is to make it so that something can allow for a realm to grow much larger without it automatically meaning that every region taken adds 1 title to attribute. Essentially, a new lordless status. But not as it is now, but one that incites realms to grow as much as possible while also inciting realms to not have more than half of its nobles as lords. This can potentially mean having some of the lordless regions fall more under the purview of dukes or rulers, that, while less effective than if it had an actual lord, is still beneficial enough to encourage realms to go to war for.
