The first thing I would do is cut everything that is only there to tease a future film that is never going to happen: Martian Manhunter, Lex Luthor and Deathstroke, the Knightmare sequence etc.
Then I would focus on making small cuts here and there for pacing, and reintegrating some small bits from the Whedon version that work. I think there’s a great 3ish hour cut of this film.