Few days ago I was massively upset about this as well, with further testing now I am developing successful methods to manage my realm nicely within 2 duchy limit, I found many tips in this forum and some with my own R&D. Here is a list of what I know so far, perhaps some of this might be something useful to you in slowly turning the tables and establishing yourself as a mighty king.
Initially I thought it would be much better not to create duchies as it brings in more problems than it is worth. Now I think different, duchies is good, the more the better as long as you let everyone know you're the king. [well I admit AI cannot 'know' that but at least you know
]
With the below tips in my current game I am 6th in the world, I am nothing like the 'Holy' roman empire which has 601 holdings but still good and exceeded my own expectations with 141 holdings in my realm and 42K troops.
* As stated above, after some painful learning progress, now I find as the King, (playing Ireland under Agnatic Primogeniture) keeping only 2 duchies in my direct control and a demesne of 6 to 11 holdings is quite possible and easy to manage.
* Without exception, each noble in the realm, shall have one (and only one) county (could be 2 or more holdings) in their direct control. Do not ever give the full Duchy with 'include lower titles' option checked, giving them several counties. Always give the first county and then duke title. Then give the remaining counties to others, as explained below.
* If you don't monitor your vassals, in time, (even if wars between vassals are prohibited) some vassals *will* expand with to marriages/plots.
* Use every opportunity to rightfully imprison any vassal that has the potential of expanding, after all, in prison they cannot make new children and cannot plot against you. (prohibiting children is a valuable tactic only for 'new noble's as explained below)
* Never grant any land to 'existing noble's, always grant land to 'new nobles' who have no family/dynasty - so when they die, it all goes back to you without incurring any tyranny or vassal relationship penalty.
* Never stay over your demesne limit, even 1 extra holding causes problems [i.e. having 11 holdings, while you should have a max of 10]. So give it away, as quick as possible, always give away the holding with least troop count.
* Always try and grant land to GOOD CANDIDATE vassals who:
+ are of your culture
+ are of your religion
+ have 'content' trait.
+ ideally, WITHOUT lustful trait, WITH celibate or at least chaste trait
+ are as old as possible, so that they will conveniently die and return your holdings back to you
Sometimes you simply can't find new nobles with content trait, and you urgently need to get rid of some holdings due to demesne size limit - in these cases, for "temporary parking" purposes, grant it to some eligible NEW NOBLE over 80 - 85 years old.
In those 80+ aged people, ensure they are NOT ambitious. Ideally they also should not have envious, deceitful traits and a high intrigue/martial skill. Basically their part in the play is to rule for a few years and then die, to perfect this, they do not need high intrigue/martial skill or any enviousness.
* Never give land to anyone ambitious. Never give land to pretenders (2nd sons etc) let them whine about it. Recommending church career to annoy close relatives is great if their intrigue is not really high. For those who have really high intrigue skill, promise some land then eliminate them with spymaster otherwise they might eliminate you. If you are taking prestige hit due to unlanded sons, (this prestige loss level is normally low enough to ignore BTW) and if this is a problem at your current level, give them one 'holding' with lowest troop count (do not give county as that will let them access bishopric/city resources as well) . Note that this is for controlling 50 holdings in your realm. If you have only about 3-6 holdings do NOT give one to your whining son(s).
* As soon as some vassal is going over the limit of one demesne, take action if possible, if not, keep an eye on it and do something about it ASAP. Do not let them grow to 3-4 holdings.
Generally it is better to have good relations with vassals, however sometimes for pruning purposes I intentionally grow my BadBoy -so to speak- high (e.g. I banish one or two vassals that were on the watch list). This angers ALL vassals but if you manage this well, the only ones that will actually ATTACK you is the ones that already had some bad feelings towards you. Which is better than attacking them and taking a hit. You get one free revocation this way (to remove the Duke title in most cases) If necessary use one paid revocation for taking the count title as well, then bring in a 'new noble' who will be happy that you gave them land, then grant duchy as well and they will adore you [+100] even though rest of the realm is furious towards you. Now you can repeat this a few times, the first Duke you just gave the duchy will not like it but his relation level which was +100 can go down only so far. Once the cleaning is complete you will have 2-3 new happy dukes, all with content flag.
GOOD CANDIDATEs sometimes do a very bad thing and produce children who may have wrong ideas (i.e. holding on to your holdings which you granted to their father for safekeeping purposes). You should clean them out before they too can reproduce and multiply your risks. The reason we go with NEW NOBLEs is that they cannot pass the holdings to anybody but you.
This style unfortunately requires assassinations sometimes. I initially did not want to use any assassination at all, or at least limit it to 2-3 times for each ruler's lifetime, however I gave up on that limitation as it is
1. dumbing down the game
2. AI *is* using assassinations, so I am starting to believe it is not such a cheap shot and more like fair game.
Hope there is something new here and you find it useful.