My theory is that since Joker 2 bombed so bad, Zaslav is NOT willing to risk another Elseworld movie. He wants momentum to make lots of money. He hopefully saw and understood the failure of The Flash and Aquaman 2.
I personally think they are trying to hold off of anything until they see the results of Superman. If it does poorly, they will go ahead and continue its independent theme, of just Batman within his universe. If SUperman does well, i think they will restructure it to be inserted into the new DCOU.
With Supergirl, so does it seem like they will have the casting all done by the time Superman comes out? And maybe the script? If so, i can see them announcing Supergirl to be the next big production after Superman, with an announcement of Batman 2 relatively soon after.