What astounds me is the fact that they handle that license well in the Dinniverse and do a passable job with it in the Arrowverse and did a really fucking awesome job with the Nolanverse. How can the same company handle the same property brilliantly in some contexts and disastrously in others? It's absolutely bizarre.
The DCAU (I think calling it the DIniverse gives him a little too much credit- Bruce Timm was instrumental too) would be an excellent model for how to do these films:
* There was a head honcho who gave the whole thing a direction and set a consistent tone
* The showrunners understood how the DC universe characters vary tonally and how to play those elements of tone and style off one another
* The showrunners had a deep respect and love for the material
* The showrunners understood also how fucking goofy comics can be, and brought that across without clashing too hard with the serious moments and
without mocking either the material or the fans
* The showrunners knew how to do a long term storyline with a payoff. BTAS and STAS were still episodic as required by television standards of the day but were vastly ahead of their time in terms of developing an ongoing sense of continuity and story, so much so that episodes of Justice League Unlimited referenced characters and plots from BTAS episodes made in the 90's
What they need to do is:
1. Get somebody who knows and enjoys comics and has editorial/management experience
2. Sit them down and let them plot out an overall arc
3. Decide which characters to feature and which characters should be introduced as side characters
4. Hire competent directors
5. Make the fucking movies
The big reason these movies are coming out so bad is lack of organization and studio thinking. Zack Snyder is good because he adapted Watchmen and 300, he does comic books, let's hire him! People like the Suicide Squad trailer more than the movie, let's have the trailer editors recut the film! Let's make a Batman and Superman movie and jam pack everything we can possibly think of into it until it becomes an overstuffed mess!
Honestly I think the whole "shared universe" thing is tripping them up. They should just start making good standalone movies. Make a good Superman movie. Make a good Batman movie. Make a good Wonder Woman movie. If they're all popular then sure, mash them up. Don't worry about continuity between movies. Find directors who suit the projects and give them room to do something creative and fun with the characters.
Continuity can be a great asset or it can kill storytelling. If the audience comes out of a movie more worried about what happened in a different movie, the one they just saw probably sucks.
See: How no one asks why the Avengers aren't in a Cap or Iron Man movie but people are asking where the hell they were in Dark World. Because Dark World wasn't a very good movie.