Black Panther debuted in 2016 CA:CW. Snyder's JL came out in 2017 with a diverse cast and a black actor as a major lead. Rick Fumiyama would have been working on the Flash solo with likely diverse cast & Cyborg solo was in the work. All these were for aught. Then Black Panther solo came out in 2018, I recalled that Fumiyama commented on the Black Panther solo along the line of "See, I told you so..." to WB. lol
We simply don't know that, had Snyder actually released his version of the film, that THE FLASH would have happened regardless of who was attached. Because there's simply no guarantee that Snyder's JL, likely compressed into a theatrical cut a la BVS would have been received as well as this four hour cut has been.
We also don't know that THE FLASH won't have a somewhat diverse cast. Kiersy Clemons is still Iris, is she not? And last time I checked, Ezra Miller is still Barry Allen.
In 2017 Snyder JL would have added apart from the trinity:
Cyborg (Black)
Aquaman (Polynesians)
Flash (Jewish via father)
These characters were added anyway, via Whedon's JL, and AQUAMAN continued the representation, as did WONDER WOMAN 2, continuing from the first one.
Along with supporting cast:
Silas & Elinor Stone (Black family)
Atom/Ryan Choi (Chinese)
J'onn J'onzz/Martian Manhunter (Black).
J'onn is black because of his human side, portrayed by actor Harry Lennix, who was already in two previous DCEU films, and arguably better featured there anyway. They already added this actor, in terms of diversity, prior to JUSTICE LEAGUE.
Silas Stone is still in the theatrical cut of JL. Can a black family not consist of two people?
I'll repeat it again: WB executives are clueless about creative diversity in film-making and about POC in general. It's same with most corporations including Disney. It's just that Marvel/Disney just took advantage, after the fact in most cases.
Are they, though? Gal Gadot, an Israeli actress, featured in BVS. WONDER WOMAN was clearly a movie with an eye toward diversity. (points at the Amazons and her team). Ditto the sequel, featuring Gal and lead villain Pedro Pascal, who is Chilean. WB also presented a diverse cast in SUICIDE SQUAD with a POC lead in Will Smith and arguably again in SHAZAM! and BIRDS OF PREY, to a lesser extent. WB has been developing BLUE BEETLE for several years. They just greenlit SUPERGIRL with a Latin actress. BLACK ADAM looks to be a fairly diverse production as well. Then you've got Ava DuVernay's THE NEW GODS project, which will amost certainly be a diverse outing.
More recently, Matt Reeves' THE BATMAN features two POC leads. We're hearing about a black Superman project as well.