JL writer: There was no plan
"Justice League writer Chris Terrio has offered new insight into how the early roadmap of the DCEU was established. According to Terrio, studio-mandated release dates, not story plans, dictated the release order of these DC movies."
https://www.ign.com/articles/justice-league-writer-criticizes-dceu-creative-roadmap
But sure, a great movie was made and Wheedon came in and single-handedly ruined it all....or something.