The most interesting approach, I think, is to flip the script on how the characters figure it out. Instead of some grand reveal or fourth-wall break, what if they start noticing tiny, persistent anachronisms? Like, Adrien's favorite book is mentioned in one episode, but he's never actually read it in the 'canon' they're living. Marinette could find storyboards in her father's bakery basement from before she was born, sketches of her own life. The horror wouldn't be immediate; it's that slow, creeping dread of 'this doesn't add up,' followed by the characters desperately trying to prove they're wrong.
Focus on the emotional fallout, not just the premise. Chat Noir would probably spiral—if his entire tragic backstory was written for ratings, does his pain even matter? Ladybug might try to logic her way into fixing a broken universe, only to realize there's no akuma to capture this time. The real conflict is internal: how do you find meaning when you suspect you're a character in someone else's story? I'd avoid making the 'writers' or 'viewers' direct antagonists; the tension should come from the characters' relationship with a truth they can't change.
For structure, a tight one-shot from Marinette's POV, maybe as journal entries tracking the inconsistencies, could be more powerful than a long, plot-heavy epic. End it ambiguously—do they tell anyone? Do they try to rebel against the 'plot'? The unanswered questions are what make the trope so chilling.