I went ahead and beat it. I don't think this is a spoiler to say, but Hyrule Castle is SUPER disappointing after the shrines and divine beast dungeons. The ending cutscenes are ok. The boss battle was easier than I expected, imo a couple of the divine beast fights were actually harder than that. But I chalk that up to gear/experience with the mechanics. I just can't get over how disappointing it is to go from one of the divine beast dungeons, which i felt could have been better but were pretty decent dungeons, to Hyrule Castle.
I'm not sure how I feel about the game right now. Amazing, of course. But the dungeon thing does bother me more now. it's hard to be too shitty about that given the breadth of the open world content, but I did expect more from the dungeons. Definitely not going to 100% it, those korok seeds can get fucked.