>>712789703
Having every single major character reject Eren's plan was certainly a massive mistake and there should've been more drama and infighting there. The plot could've been used generally as is minus all the bullshit about Eren planning to lose and Ymir mindfucking him(which both invalidated all the good writing from 20 chapters ago between Zeke and Eren). It was this character assassination of half the cast that really ruined it, not the underlying story beats.
Or he could've gone for a melancholic genocide ending where Eren wins after killing a lot of his comrades.
He chose the cheesiest and laziest way to conclude it after previously demonstrating he was capable of writing far better.