If costumes are tied to full galleries, it's probably worth considering switching the acquisition method to something that the average player can be expected to reach. You've already got the extra costumes for highly committed players that want to clear every difficulty, but just getting one costume should be something that anyone can accomplish. If you're thinking of switching to cumulative points, you could just put all of the rewards on the same track and have the difficulties give you different score multipliers, which would allow players to get things at their own pace. The current system incentivizes trying every difficulty regardless of personal skill, which can make the hard rewards unreachable for low skill players, while the skilled players must face a slow grind to get the easy rewards. I think that your idea of making this into more of a coffee-break game is a worthy pursuit. Let players go as hard or as slow as they want, with something to show for their efforts.