Thanks a lot for checking my game out with such detail!
I understand that if you make a cluster large enough, you will end up with a structure that prevents you from using all the tiles. I'd like to say that's a design choice because if I were to allow players to fill the entire grid that easily, then the player will end up with a literally invincible defense line and make the level extremely easy to clear. So the strategy is to combine tiles into clusters only where necessary and keep some tiles as a sacrifice.
But if it ain't fun, there's no point. So I appreciate the suggestion!