They could have introduced each of these other characters slowly over the course of, say, four films: two Superman, one Batman, and one Wonder Woman.

Each character didn’t need their own film to build to a JL film, just the three core characters.

Introduce, say, Aquaman in Supes 1 and give him an increased role as an uneasy ally in Supes 2, possibly with WW and/or Flash in, again, smaller roles.

Introduce Cyborg in Batman with a pre-established relationship between Batman and Flash, with Batman serving as a reluctant mentor of sorts. Instead of Lex keeping a dossier on everyone just set that in play in the Batman movie since it’s true to his character anyways. It wouldn’t take more than, say, 15 minutes of screen time woven throughout the film to establish these things and the film would still center on more traditional Batman stories.

Put Aquaman in a small role in WW, perhaps assisting during an ancient battle and establishing some kind of relationship between Atlantis and Themyscira and utlize Aquaman as distant but reliable ally to WW.

Add small elements here and there to hint at Darkseid’s arrival and the requisite post-credit scenes and at that point we’ve established a firm foundation for everyone coming together in the first JL film. For example, a better-executed version of the Knightmare could have been played out in Batman as a post-credit scene. Give Aquaman the post-credit scene for WW, things like that.

I’m sure you can poke holes all throughout that idea but I think something along those lines would have made a sound framework for JL and the DCEU. I don’t think everyone needed a movie, I just think they needed a stronger focus from the beginning. They should have peppered Flash, Aquaman and Cyborg into four films based on the Big Three and given them increased roles in the first JL film instead of the rush job done in BVS