Solo had its first teaser trailer 3 1/2 months before its release. Sure, that wasn't during COVID times, but there was a LOT of criticism for the late teaser. That film, while not horrible, had SO many problems throughout production. And those problems led to a poor marketing plan and horrific box office results. Another potentially "big" movie that had a very late trailer drop was Dark Tower. That one was worse.
With 7 movies coming out between July, 2021 and July, 2022, there is going to have to be a LOT of overlapping promotion. Even if the most recent schedule changes weren't made, it's still FOUR MCU films released just in 2021. They cannot at all wait to promote one at a time. If so, they'd release a Shang-Chi trailer 2 months before release, an Eternals trailer 2 months, and Spider man 1 month.
What troubles me is that we've heard absolutely nothing about this film. We've heard buzz about Eternals and have seen the cast pictures from conventions (albeit not high quality). But there hasn't been a peep about Shang-Chi. And it wrapped filming a while ago.