There is nobility privilege allowing further decrease of liberty desire. It also grants +2 diploslots, but requires you to have 2 vassals. look around who can be vassalised or maybe there is someone you can release (for example take Istria and Friuli fromVenezia, release Dalmatia and Aquileia). Having vassals also increases your force limit, so you can build larger army, what would also decrease subject's liberty desire.
if you go to war, make sure you don't lose troops, let your subjects do the job and waste troops, increasing ratio between your and their strength
being behind diplotech would cause you suffer 5% additional liberty desire
hire diploreputation advisor
Austria is pretty managabe with these tricks.
Once you get experience, play as Bohemia and enforce PU over Poland having Lithuania under PU 15 years after game start. This is hardcore
