I went into the castle looking for the memory there since I had all the other ones. Was not particularly prepared for anything, and intended to get in, get memory, then get out. Ended up exploring like the entire castle, found the Hylian Shield, saved by Mipha's grace a bunch of times, before finding where the memory was. At this point, I had cleared a handful of guardians out of the castle and was still pretty new to guardian combat, used the ancient arrows I found in the castle on a bunch of them, so to save the effort of trying to brute force my way through again later, I decided to blindly attempt Ganon.
At first I was confused because Fireblight Ganon was the one who greeted me, but I caught on quickly. I managed on my first try, though after Calamity Ganon's second phase, I pretty much resorted to spamming bomb arrows with the Great Eagle Bow. Calamity Ganon's fight was also the first time I ever thought to utilize parrying, since I stopped using shields long before that fight.
I remember staying up 'til like 8 AM because of storming the castle and all that momentum making me go attempt Ganon. I never went back to do the other two Divine Beasts in my first file after beating Ganon without them.