The Flash is going to be a time travel movie (Flashpoint for us comic nerds) where they meet Michael Keaton's Batman, then whatever happens in the movie will swap him for Ben Affleck, that's why Keaton and JK Simmons in Batgirl.
Warner didn't want to tie Batman down to just appearing in Justice League films so they were going to have Affleck as his JL "present" version (along with cameos) and Reeves' stuff as set in the past but then the JL stuff all collapsed, Affleck decided he didn't want to do the role anymore (especially since it'd be 5+ years since he last played it) but would do it again for Snyder and The Flash movie. After the Snyder fall out and Joker made $1 billion they just decided to have a "multiverse" and allow for all these movies and stuff set in things outside the main DCEU.
The DCEU will still be going with Gadot's Wonder Woman, Mamoa's Aquaman, Miller's Flash, etc. But apparently with Keaton as Batman. And while Cavill still wants to be Superman I have to imagine they want to swap him. It doesn't really matter since they won't be teaming them all up as the Justice League for some time. I figure they'll swap in a new younger Batman at some point to replace Keaton when they actually want to do another Justice League movie. (Although at that point they might just recast the entire team.) The DCEU will still be a shared universe but I don't think they're going to be doing anything like Marvel where everyone appears in every film and it's one ongoing plot like Marvel did from Civil War on up to Thanos. It'll just be references to others and maybe some cameos, maybe some Team-Ups where like you say have a Aquaman and Flash movie or something but not really the rest of the team except maybe Keaton doing a cameo as setup or a voiceover "from the Satellite" or whatever. Black Adam will setup JSA characters but I doubt they appear much outside of those and the Shazam films, those two will obviously cross over at some point.
Snyder's Justice League trilogy idea left some leeway at the end of the third film for Warner to start recasting the roles like Batman especially. I've had a similar thought for some time, DC has Crisis where it can fall back on "rebooting" the multiverse in a way that's actually faithful to the source material.
I like DC and like the Justice League so I want to see them as movies still but they probably really aren't going to do that for any time soon so without that thread tying the whole thing together the DCEU being a shared universe is just references and cameos rather than story arcs. The funny thing is Snyder was planning on giving them exactly what they wanted: big event movies with 2 and 3. But the former executives before AT&T didn't want to wait, they wanted it NOW and in two hours with Whedonesque dialogue! And that wasn't the guy they had hired, so they dumped him to try and get it on the cheap. Oh, well.