"I'm pretty sure you only get decadency from direct vassals and courtiers. Every time one of my uncles dies I have a lot of unlanded relatives in my religion tab.
IN any case, one Jihad or Invasion will give you enough land to take your decadency from 70% to 20%. The real problem is, if you don't have enough relatives, you'll wind up with some super powerful Emirs. "
Start a game as the Seljuks, look at your religion tab and see where your unlanded nephews (who are generating the most decadence) are located. Now grant them counties that are de jure Baghdad and transfer their vassalage to the Caliph in Baghdad. Are you sure that only direct vassals and courtiers generate decadence now?
To the OP, congratulations you found the simplest solution to decadence, keep conquering (if you keep playing don't stop or your large overpowered empire will collapse). The only problem with this is that if you conquer the world too early you don't have any more land or titles to give by the time the Mongol Hordes come, assuming your dynasty even survives until then (I would recommend fighting your relatives since even if they win your dynasty survives). If you like playing with "game mechanics" then I hate to say this to you but this is the wrong game. Everything is overpowered for the human player (unless you shackle yourself with house rules like most of us who still play the game). Using "game mechanics" you can go from a one province count in HRE to Emperor of most of the world in the same generation. World Conquest probably by the third generation, though making it stable enough to last until the end of the era and surviving the Mongol Hordes is harder. Do you really want to showcase the overpowered Muslims without any restrictions? Start as a one province sheik in Iberia and hire mercenaries. Now just keep declaring war, you should have most of the known world by your third generation and if you don't lightly dismiss decadence then you can probably survive until the end of the era and beat the Hordes back.
If you find that sort of thing fun start as William the Conqueror, or Harald of Norway at the starting scenario. If you don't end up Emperor of most of the world or world conquest when your second ruler dies you were probably unlucky.