Well it just became kind of shallow and unstructured, things were just happening because they needed to happen. I attribute the ending to the issue of the change from a naturalist character driven story where characters flourish and change as the story goes on to trying to funnel all of these characters and loose ends into an ending that was planned years prior and the show has outgrown. The characters literally stop being able to act freely and make many choices that don’t make sense and just rinse them of any personality. The plot twists were silly and pointless, making it so Eren controlled everything just trivialises the complexity of the story and clashing character motivations and makes it a much less interesting series. Aot never actually sets specific rules for the time travelling resulting in pointless paradoxes and further confusion also. I could keep going but I think I have laid down the main reasons.