I believe that categories should be defined by in-game definitions, and I think 1st credits is a good enough definition to say that you have "beaten the game".

Yes, the game certainly shows the credits after beat chapter 4, so I would like to chapter 4 to be Any%, and chapter 5 to be Best Ending. This matches other modern titles (Blowout Blast or Kirby Clash). I think both would be good main categories.