There's honestly not a whole lot I can point to to improve this one. I think it's a really solid all around entry. The "dungeon" seems really straightforward to run, well-written, and has a diverse array of encounters that all fit the theme. I really like the melding of lovecraftian horror with the fantasy fae realm setting.
If there was one thing I could point to to add, maybe play up the alien weirdness factor of the island itself. Make the island feel like it came from a different world as the adventurers explore it, but even that can be easily handled by a GM on their own.