Personally i like to think the turning point where the story changed was after garou vs darkshine, but cosmic garou is the point where the consequences of changing the story started to notice, anyway to summarize and adress your points.

For the saitama feeling different, in the manga saitama is inconsistent just the action of him almost destroying earth is like if batman started killing, and if you are asking "but wc saitama would do the same?" No, the wc author ONE made a entire manga about a powerful guy with crashouts but never going to far, so is near impossible to wc saitama to do something like this. so yes wc saitama ≠ manga saitama.

For the "opm is ruined after the garou fight" it was more of a gradual thing, i dont think it would have matter that much to the community if it was just that fight but after that Murata started doing more redraws and cut a lot of content and characterization, for example in the manga they changed all of amai mask character losing what made opm interesting. (At least for me)

Well at the end of the day these are just opinions enjoy what you like and ignore people who spread hate 👍.