Thank you very much for this detailed comment :D
Indeed, the bosses have some patterns that evolve when they are selected late, like the joker and his wheel of vulnerability that becomes more and more complex.
As for the attacks that persist when the boss dies, that was a deliberate choice. Fish persist, so does the magic hat rabbit, and so do the playing cards. I wanted there to be a certain porosity between the bosses. Logically, these elements have time to disappear during the transition scene between bosses (unless you skip the scene).