Whether or not you "buy it" bears no weight on the truth.
WB was hellbent on releasing Justice League prior to any consideration for Man of Steel 2. Zack Snyder always intended on creating a 3-part Superman story. He almost accomplished this - if not for studio interference. Justice League is clearly a Superman movie. Everything from the beginning and credits, the entire convergence of relying on his resurrection, his entrance into the 3rd Act... It's all Superman, so much so that Whedon couldn't cut (or add) enough of his content to make it more of an overall team movie.
Justice League was written/designed as a 2-parter because of the Superman Trilogy. Snyder wanted to do the team justice, in a fashion, by actually showing off the full team dynamic in Part 2. Regardless of whether or not that works for you, that was Snyder's compromise of character/content for having to compromise on his Superman story. I'd bet that most of the things you don't like, you probably blame on Snyder, when that's just deflecting it from the true cohorts (WB). This isn't to say that Snyder has no blame to receive, but he gets flak for many things that he just isn't responsible for.
You also have a method of character expansion after MoS, too. BvS cameos them, JL introduces them, and JL2 was to show them off. There was tons of character content written, executed, and cut from JL. After those intros, it only makes sense that you can 'show them off' as equal members in the sequel. Snyder was the only one at Worlds of DC with a plan. It wasn't perfect, but it was different, and it was something. Now WDC is basically a headless chicken.
Part II