Just running through the events of CoIE in my mind I guess you could leave out;
- The Freedom Fighters and the Charlton heroes
- Sgt Rock/Haunted Tank, etc
- Jonah Hex, Bat Lash, etc
- The prehistoric stuff
- Kamandi, etc in the future
So no need to travel back/forwards in time, the story can be kept in the present, focusing on characters already known to the GA - so basically all those appearing in DCEU movies up to that point and alternate versions of them from several Earths.
Also, CoIE suffers from serious ending fatigue ('We've beaten the Anti-Monitor! Oh no, there he is... Now we've beaten him! Oh wait, he's back...' and so on), so that can be pared down to one big battle towards the end, followed by an epilogue showing the changes resulting from the whole thing (maybe with a voice-over by Harbinger, along the lines of Amy Adams at the end of JL).
That's just off the top of my head. I think you could still get a sense of massive scale but make it easier to follow.