While you make some interesting points, I still think you severely underestimate the boost such a big cast would give to the film's marketing. The delay can be both a tactic and a bit of a necessity. One doesn't necessarily negate the other.
I mean if they only wanted to scare actors they would have moved their entire slate like Sony. Instead they moved only the one big movie we know they care enough to do well for future installments and they left Aquaman, from a dying franchise that they couldn't care less, to rot.
The biggest problems the sequel faces are bigger marketing cost due to the delay and losing a bit of the momentum (ideally it should have been a movie that came one year after Part One, instead it will come after two and a half years).
I still believe that it wouldn't have any serious chance for a solid Oscar campaign without the actors to support it, and even though March generally is not a good month for awards contention, like
@Drizzle said it still did wonders for Everything, Everywhere, All at Once.
Furthermore competition in March doesn't look as severe as having a big Marvel movie opening one week after this film's premiere. Biggest competition is Snow White and all signs show that it will be delayed too, as will other movies early next year (not that I think it will do serious numbers like Little Mermaid in the US).
As for the IMAX screens I don't see any reason why it won't get another exclusive window if it got one the first time. Even if it gets a smaller one, if the movie is a big hit it will extend like Oppenheimer's.