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In addition to my post above about getting the good ending. I could send you my save file to check if at least the DLC works.
Is this really, truly Add-On content, or is this something I NEED to play if I really want a real, satisfying conclusion to CrossCode?

Because, an estimated 50-60 or so hours just to complete the main story campaign sounds pretty daunting as-is; can't imagine a game requiring that amount of time just to finish it, and still wanting more afterward.