It's not really about sustained creativity, the anime just dragged because they wanted to milk the franchise as much as possible. Having like 4 seasons named the final season. They adapted a 139 chapter manga into nearly a 100 episodes.

It doesn't help that Isayama himself wasn't sure how to reaolve Eren's character, he originally planned a tragic ending where everyone diea but changed his mind due to the popularity of the series. The timeskip and Eren's 180 degree personality change was the biggest issues, it was implied that due the abilities of the Attack Titan he saw something and had some motivation to change. Unfortunately trying to write a reasonable motivation for the MC to commit genocide is pretty much impossible so the ending ended up being a cheaper and worse executed version of the Code Geass ending.

DBZ (I feel like they fight multiple gods)

Yup. DragonBall Z and Super both kept adding higher and higher echelons of deities and otherworldly beings, starting with the Kais, right up to a being that sits over multiple universes. Of course, Goku tries to pick a fight with every single last one of them, with very little regard for the existence of multiple of said universes.

Seven deadly sins

I'm gonna be real here: I have no idea if that tracks or not, and I did my level best to pay attention. The story makes so little sense, with all the "rules" getting thrown out and replaced with more nonsense, every five episodes. They easily could have wound up at a lower power scale at the end than when they started, and you'd have no way to know for certain. That said, they do drag top-dog spiritual entities into the mix near the end.