The whole DCEU as an idea was flawed almost from the very beginning. They just rushed everything through without really seemingly thinking at all about setting up anything beforehand. That's why they basically made every other movie a team-up between characters without ever really setting up most of them in any meaningful way. Justice League especially was silly, as it was like Marvel making the first Avengers film immediately after the first Iron Man. That just doesn't work.
As far as Cavill leaving the role, I don't really blame him. He's a decent actor, but the way they handled Superman gave him nothing to work with.