My thoughts are in blue, words to delete are in red, suggested rephrasing is in orange.

Anne lives on a remote and dilapidated island in the Mediterranean Sea, and she wants out. My immediate question is why? Her age becomes clear in the next sentence, but I'd move the fact that she's a teen into the hook The moment she turns eighteen, Anne plans on buying a one-way ticket to Corsica, leaving her oppressive life what makes it oppressive? Lack of options? Or something less benign? behind, and pursuing her dream career as a journalist. But when all two thousand of the island’s residents begin undergoing gruesome and terrifying changes, Anne is forced to reconsider her plans. What are the changes? Are they turning into chickens? Or are they bleeding out of their eyeballs? Why would this make her reconsider her plans? Wouldn't that be more of a reason to leave?

Surviving the apocalypse proves to be quite difficult, especially when each islander has devolved into a grotesque embodiment of their greatest fear. I like this! Say so in the previous para. Also - why is Anne not affected? With no way to contact the outside world, Why not? Anne assumes that she’s the only human left. That is, until she runs into a group of four survivors. Despite her inhibitions, like what? Anne decides to join the group, and she even befriends Vider Gris, a mysterious young man who always wears a leather glove on his left arm. Hand, or arm? Anne has finally found something worth staying on the island for—a friend. Earlier it sounded like her reasons for leaving were career and opportunity driven, not out of loneliness.

But as Anne’s friendship with Vider grows, so does her suspicion of the group’s leader, Ralin. Even in the midst of an apocalypse, all Ralin wants is control over Vider—and he’s willing to do whatever it takes to get it. Why is Vider so important?The way this is written makes it sound like Ralin was desirous of this control even pre-apocalypse Anne only realizes the gravity of her situation when she discovers what Vider has been hiding under his glove. If she doesn’t do something about Ralin, she’ll lose her first and only friend, and the apocalypse will be the least of her concerns. I'm torn here b/c typically you don't want to tease in a query, but I can see why you'd want to keep the glove reveal under wraps. But it also seems like the answer to that is way worse than an actual apocalypse needs to be shared in order to convey the plot. I'd say you need to share what's under the glove so that it's clear what's at stake.

THE ISLAND OF TWO THOUSAND FEARS is a supernatural horror novel that would appeal to fans of BIRD BOX with its apocalyptic elements and strong female protagonist and THE FISHERMAN by John Langan with its emotional and character-centered plot. The manuscript is complete at one hundred thousand words. This is long for a YA, I'd try to get this pared down to 90k

I have experience marketing creative content as I once owned a YouTube channel with over three hundred thousand subscribers. I drew inspiration for the unique setting of THE ISLAND OF TWO THOUSAND FEARS from my personal history and culture, as my family is French, living predominantly in Brittany, Paris, and Corsica. Good bio - you make it clear that you will be a boon in the marketing arena, and that you're qualified to write about this setting. If it's still in existence, I'd say what the YouTube channel is, and if the content of it was relative to this book at all, I would say that as well.