The best path seems to be setting it up like the MCU by separate the universe into Phases that end with Justice League movies.
I would restart the DCEU. Man of Steel was a good start to the DCEU, but Batman v Superman could have been saved for later. I would have introduced at least Batman, Wonder Woman, the Flash, and Aquaman.
Then, in the Justice League movie, I would have introduced Cyborg with his mother box origin story. The new Justice League movie seems to going down a decent path with Steppenwolf as Darkseid's lieutenant hunting down three motherboxes. A Green Lantern tease would be cool.
Phase 2: I would have a wave of sequels for the aforementioned superheroes besides Cyborg. I would not have a Black Adam movie, but just a Shazam movie, and maybe a Green Lantern Corp movie with Hal and John. Man of Steel 2 could have some Batman, as well as introduce Supergirl and Brainiac. Throughout these movies, Lex Luthor would recruit the villains (not Brainiac) to the Legion of Doom.
Then for Justice League 2, I would have a power-hungry Lex Luthor, who is promised power by Darkseid, lead the Legion of Doom to take down the Justice League. At the end of the movie, Supergirl, Shazam, and the Lanterns would join the League.
Phase 3: I don't really know. Some original stories would be cool, but also stories like Court of Owls would be awesome. Bring in new charavters, potentially Martian Manhunter. Maybe have three-quels. Maybe have the Suicide Squad running quiet government operations, not huge metahuman events.
Justice League 3 would be like Avengers Infinity War. Darkseid is a God, and a roster of superheroes have to take him down. Maybe invite the Legion of Doom and other villains. Darkseid cannot and should not be nerfed. Kill Batman/send him back in time like in Final Crisis, kill a Lantern, kill some villains. It would be a huge event, potentially even two parts.
Phase 4: Battle for the Cowl, Blackest Night, Return of Bruce Wayne, Under the Red Hood, Endgame. So many possibilities. Crisis on Infinite Earths would be terrific to make Wally front and center instead of Barry.
As far as casting goes, I wouldn't change much because most casting decisions have been good. I would have an older, more sane Lex however.
Of course, I don't know how practical this would be since I am no filmmaker.