I didn't mention Prim. And it doesn't have to be rigid to still maintain some sort of narrative. It's just not very interesting story wise, there's no motivations for the MC that I can get with. Even disregarding the MC, the story for each girl doesn't really have any direction really. Maybe Yui has some mystery that can slowly be unraveled, but the others don't really have much of a meaningful story and you don't really provide a reason to care.
Nonsense! I can't speak for Charlotte because I didn't wanna bother with her...
Layla has the whole "mad scientist trying to unravel the mystery of the town" plotline:
Prim has the restaurant development and her desire to improve her gaming skills-- she's not a functional mute with a dark backstory a tsundere scientist, but it's not a bad aspiration or plot.
I honestly can't tell you what's up with Maxy, but that's sort of her charm; she lives flying by the seat of her pants. She doesn't seem to have much of a plot-based reason, but that's why Maxy is who she is I think. Her family connections could bloom into some plot later. One that intersects with Layla's and/or Yui's.