that’s the thing though ,she starts TLJ in a place of doubt , " I need someone to show me my place in all this ." In the end ,she has cast that doubt aside .She literally has no time to doubt or question her abilities in the moment where she lifts the rocks or people will die . I am not sure what you meant by it happens off screen . If you are referring to her thoughts , I suppose that could be the case but I don’t see how that is relevant . other than comparing it to Luke expressing doubt about lifting the x-wing . And having her use two hands would be to weaken her character in my opinion .
The moment is at the climax of the film, and should be the climax of her arc. Everything has come to a head here. She’s starts the film with doubt, yes, about her “place in all this.” But her mission throughout the film is to find the person who will save the Resistance. First she looks to Luke, then Ben. Ultimately the truth is that that person is her (which is also the answer to her place in it all). That realization should’ve happened in that spot, with her coming into the scene doubting herself and the fate of the Resistance after her failure with both Luke and Kylo and having no choice but to assume the mantle herself, which she isn’t prepared for. Instead the realization has either happened off screen or simply has yet to happen at all. I suppose Rey coming to terms with that understanding of her role will likely be part of her story in IX.