As others have noted, the Mamoa recasting was absolutely a matter of correcting a massive casting mistake in the first place for something that Mamoa is actually an insanely good choice for (say what you will about the Minecraft movie, thank you I will it was dreadful, but it was also probably the most compelling Mamoa has ever been, and it was definitely a more Lobo-adjacent role). Mamoa as Lobo makes all the sense, as much sense of James Gunn bringing Lobo to the screen in the first place, and Gunn's already displaced a willingness to play with the existence of the previous DCEU in this new one.
Gal Gadot is a terrible actress, first and foremost, all other potential controversies aside (she also got the biggest flak from being the one to create and share that absolutely dreadful "Imagine" video during the pandemic. Did you know Natalie Portman was in that travesty? So was Pedro Pascal!). She cannot act. Now, and this is the trick few people are willing to acknowledge, sometimes being a terrible actor is a good thing. There's a certain type of character it kind of works for (think David Boreanaz in the first season of Buffy). This is why I think, by and large, her first movie ended up doing well (beyond her looking perfect for the part, Chris Pine, low-key the best Chris, doing everything he can to salvage it, and the general vibes in the world kind of needing it to have been a good movie, I'd like to think with the value of hindsight that we can admit that it was actually pretty bad). But in general, yeah, she's a terrible actress. Don't cast her in things.
Ezra Miller comes with their own massive can of worms, so they're also out. Cavill definitely isn't coming back, and honestly good for him. I could see Ben showing up again in some minor role, at least. Maybe some supporting cast, like Irons or Fishbourne might pop up again. And if we're very, very lucky, we will not see Jared Leto on screen ever again.