Why even bother with a script?

You could just assemble a cast, bring them into a greenscreen studio, and shoot a complete matrix of poses and angles and lighting variations. Then synthesize the film from that when someone decides what it should be about.

They even did fakeouts by the trailers using even further different CG costumes.

Some of the reason they gave for that at the time wasn't just saving on real world costume designers by outsourcing it to "cheap" VFX teams, but also for added "security" to avoid "spoilers" leaking to the press in tabloid photos.

(Spoilers of a sort: My favorite meta-joke on all this was Spider-Man: Far From Home's "primary" Mysterio costume which had tabloids and fans doing all sorts of speculation what the CG composited costume on top of it would look like, given the Avengers situation, only to find out that was actually the outfit for many of Mysterio's scenes because it very well fit the character in that medium.)

This is ridiculous and completely untrue. They filmed the actors in special suits that could be tracked more easily. They added the suits as CG because the CG suits look much better and let the actors actually move around. Not only that but if the iron man or war machine masks go down and obscure their face they can transition to a full CG character and have them fly off.