With hindsight, I think this is a case where going big right out of the gate might have worked out better. Rather than doing a big build-up to Darkseid and the Fourth World, just start a Justice League movie with that. Series of booms all around the world, Parademons flood out, clearing the way for Darkseid, each member of the League is introduced trying to deal with it on their own, then they come together to take on Darkseid. Tease the New Gods by having some of the team captured part way through and Mister Miracle and Big Barda help them escape (and deliver exposition).
It’s probably 85% the first Avengers movie, and works a bit better if you don’t have to spend time resurrecting Superman, but I think if you throw it all on the screen (as Aquaman did) the audience recognizes that.