Thanks for the kind words :) no, there's no secret 6th ending.
You're absolutely on point regarding the grind to get all the endings. I originally planned to have some kind of "new game+"-option after beating it once to start at different stages of the game to make it easier to get all the endings. But that's a technical nightmare. The engine is a cobbled together mess from the original game jam. And implementing some kind of state management was a huge endeavor which I was not able to tackle without refactoring half of the game. So, currently, there isn't any other option than to start the game over :/