Congratulations!
I regret to inform you that in a few days the game will be getting an update that will make it generally easier to achieve, perhaps I'll make saves not transition over however so you and any others like you can flex! Sadly that would mean that everyone would have to start from scratch.
I must mention that this isn't the same game I made years ago but a remake of it's original version which is why it holds up(it's only a few weeks old as of writing this), I do intend to publicly release the original builds again however but be warned they are worse in every regard!
As for an ultimate unlock for completing everything I've wanted to do that but the intention was to see if I could make it within a week and decided that I wouldn't go too beyond the original game's original version, plus I was already pushing the time limit with Trixie's fight which is probably why it's turned out how it did.
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Perhaps, you don't need to really make a secret boss fight for no hitting all the bosses in all difficulties in the game, but something like a golden Ditzy Doo sprite on the title would be good enough as well, similar to how UCN has Freddy statues. This is still a really good game though honestly! A underrated gem!