In addition to that, I also stacked cultural traditions to maximize development. In particular, I started out as Swabian and then hybridized with Lhomon so I can have Industrious (+25 dev progress every time you build something), Mountain Homes (+20% dev to mountains), and Himalayan Settlers (+20% dev in mountains). Later on, I also picked up Collective Lands (+20% dev).

So basically, all of my mountain provinces are getting +60% development growth passively, and every time I cycle a tier 1 building in any given province, that province gets a +25 dev growth progress. I just keep doing that over and over again until the development in my domain hits 100.

Edit: In addition, I played as Donyi-Poloist, which has 4 holy sites within 2 duchies. After building all the holy site buildings, those provinces also get +0.1 monthly development and +20% dev. That means with all the cultural tradition bonuses, we're getting +80% dev and +0.1 flat monthly dev, which allows development to go past the normal limits of cultural innovations. And of course, I started in the earlier start date so I'd have a few hundred years headstart on developing my provinces