The turning point for bosses where you start needing to be reactive and start being focused for bosses I feel is when you first find out the name of who your attacker is as after that you get dropped in it. You'll know when it happens and the is no going back after that.
I earlier said the game is like SM/Fusion but now the game has become Super Fusion with some interesting QOL changes for certain abilities and so far in my top two of the series I'll also say that the ability they showed in the trailers you'll know when you get it is probably the single best ability Samus has ever obtained in the series.