Actually, something that I can address right now because I've completed that part, it's the fucking furniture.

What the fuck IS this? Juste finds an empty room in the castle, and having found some pieces of furniture or other trinkets around the castle, he decides to decorate the room.
Setting aside how absurd this is from a storytelling/plot perspective, it feels like this is just such an odd thing to put in a game from a gameplay perspective. There's an uncited claim on the wikia page that this was supposedly a concept intended for Symphony Of The Night, but which never made it in, and they decided to have it in Harmony Of Dissonance, but that doesn't explain anything.

It could almost feel like there was supposed to be more to this concept at some point, and then was simplified to just collecting, but at the same time it also feels like something they could have just put in for the sake of having the player gather collectibles.
In other types of games, I could see this for something like a player character's home, where they'll always have that and there's this logic to collecting furniture and items for your house, which the character will then have after the adventure is over, but in this game, presumably the room and its interior decoration vanishes together with the castle at the end, which makes it feel yet more pointless.