You also get +5 diplo rep during the Age of Revolutions if you have the MoH DLC.Every time your ruler dies, there's a chance for him to inherit a PU minor directly.
Spain is huge, so you'll only get to a single-digit chance.
+2 from NIs, +1 from being Emperor, +4 from ideas, +3 from policies, +1 from your advisor and a chance for +1 from an event, +1 from a mission and +1 from Charismatic Negotiator.
The last three being semi-random. Remember, traits are weighted and Austria gets charismatic negotiators more often.
Can't remember the numbers, last time I checked was in my Saxony run and I didn't focus on it, but it was something like 150 dev per dip rep.
Long story short, everything up to 1500 development is easy to inherit as Austria.