If I had to make a guess, Zhao probably came up with a really good overall story outline. . . that would have taken four hours to actually execute, because it effectively had two entire three-act structures. Trying to turn it into a single reasonable length movie is the source of most of the flaws. Why is Sersi so bland except in the intro/outro? Because her role got cut to the barest functional bones to fit everyone else in. Why the two bits of rather bizarre and clunky exposition? As a patch to fit in backstory and reveals that they just didn't have time to include naturally. Why are the Deviants so irrelevant and their boss almost nameless? Because they were the Big Bad of the first Act 3, and they had to cut out pretty much the entirety of that from the movie. Etc.
For reference, my guess about the original story outline:
Act 1- Intro to the main Eternals in the present day and arrival of the Deviant threat
Act 2- Travel the world, gather the gang together
Act 3- United, face and defeat the Deviant menace, but in doing so learn the threat is not that simple
Act 4- Investigate this new info, discover uncomfortable truths
Act 5- The big reveal about Celestials, with Ikaris exposed and the Eternals divided
Act 6- The Eternals form up, side against side, and fight over the fate of the world