OK, having finished both true ending and normal end, I can say confidently that the game 100% expect you to do normal end first, and a lot of the game is calibrated toward it.
I would also say that doing the bad end (or at least a run where you have the option to do the bad end) first is also probably not how you are supposed to play, simply because it more or less dismiss the whole balance of the boss battles, and this game actually seemed to have put thoughts into those.
But yeah, while It's 100% doable to do true end on hard first (I got to demon castle that way, and only put it to easy there because each fights became a non fun slog), it's also definitely not how the game is supposed to be played.