I love this whole essay, but just want to add my thoughts on another reason Liz is portrayed in a somewhat shallow way… please be aware that I’m assuming anyone reading this has already seen the film. (SPOILERS)
First, the film primarily shows many characters, including Liz, as Peter sees them, either in class with him or in a setting he’s supposed to be at but is absent from because of his dual life. Peter is the POV character for this film, and really only Toomes has characterization that we see happening outside of his presence.
On top of that, for narrative reasons it’s important not to reveal her life outside of school. We can’t meet her parents early, or even be given her full name because it’s important that the audience learn about her family at the same time that Peter does. I think the decision to tell the story in this way requires limiting information about most of Peter’s classmates. We know Michelle likes to be called MJ, and that Ned’s best friend is Peter Parker, but we know nothing more of their full names, home lives, or family. To limit information only about Liz is to draw attention to it and ruin one of the biggest dramatic moments in the film.