I don't know if the game was updated, but I didn't find it overly "grindy." I also did n't unlock my final slot until the last level anyway, so I didn't see a key issue. If this was because the game was updated, then your fix worked. If not, then some players need to employ a better strategy when fighting these levels, and you'll get through them faster with weaker units
Consider adding a button that makes the level all 2 star or all 3 star. When I'm going back through to get my silver and gold, it's tedious to keep checking every box. Otherwise, well done. I'm not finished yet, but some difficulty could also be added. 5/5
I appreciate the nice story, but the medium may be wrong. You might be better served writing a book, or even an animated story. As a game, it's too much for my taste, but it is a good story. Consider taking it to another medium where it can be truly great! I will finish the game, as the story is that good, but need to do it in small pieces as the text is straining my eyes
 "Brain Diet"){kind=icon}
 "Me And The Key 2"){kind=icon}
 "Wonderputt"){kind=icon}
 "Brave Shorties 2"){kind=icon}
 "Crown Dungeon"){kind=icon}
 "Symbiosis"){kind=icon}
 "red"){kind=icon}
 "Balls vs Zombies"){kind=icon}
 "SAS: Zombie Assault 4"){kind=icon}
 "Exit/Corners"){kind=icon}
 "Aground"){kind=icon}
 "NoNoSparks: The Ark"){kind=icon}
 "The Illusory Wall"){kind=icon}
 "Giants and Dwarves TD"){kind=icon}
 "Kamikaze Pigs"){kind=icon}
 "PUSH"){kind=icon}
 "Pixoji"){kind=icon}
 "Hedgehog Launch 2"){kind=icon}
 "Castle Crashing \"The Beard\""){kind=icon}