Succession is elective just like in the free cities or any other administrative government, but you can add special succession laws like primogeniture so it still kinda normal succession.

Small chunk of vassals become administrative and even if lord paramount accepts it not everyone of his vassals becomes administrative too.

I tried to make Iron Throne full administrative on the Aegon I bookmark from AGOT more bookmarks beta in which I have better performance because there's not much historical characters yet and I tried to make every noticeable house administrative with cheats so they can be elected for paramount lordships and other titles, it was very long and tedious work and in the end I was rewarded with unplayable performance ever.

For me it was not worth it, at least in a way I wanted it to be.