Well, looking at James Gunn’s last few forays into the superhero genre, usually the big moment is more of an emotional motivation for the main hero to gather the strength to get back up/stand up against the main threat/villain near the middle/end of the third act, than a strictly physical feat of strength.
So, I imagine it’ll be a moment near the end of the film. Hypothetically, maybe Superman has really taken a beating and is really struggling to get back up. Then, a voice, let’s say it’s a little kid, starts encouraging him to get back up and that he’s got this, or something like that. Then, gradually, more and more people in the city join in, until it finally becomes a deafening cacophony. All of the support from everyone in Metropolis is enough for Superman to dig deep and muster the strength and will to get back up and keep going. (Hell, if you want to make the moment even better and more impactful, have whatever is happening with Superman being livestreamed or something and everyone in the entire world is watching and joins in on the chanting.)
I think going that route instead of just having a showcase of strength would be a great contrast to the previous film portrayals and really show what makes this Superman so different and great. And, from all of the set pics we’ve seen so far, they’re making much more of an effort to make Metropolis feel like its own character and having Superman become more connected to it and it’s people by becoming one of them. It’s not just about his own power and strength, but also about the emotional power he gathers from those he saves and who support/love him to do what he does. That’s Superman.