I like the simplicity of the design, stark but elegant and on theme, easy to follow and very legible. Map is a little difficult to read but you also don't *really* need it. My biggest problem with it is I'm not sure I buy "Caliban’s cruelty was his passing."
"The island I love is going to suffer now that I'm dead." is how I took it, please correct me if I misunderstood.
Also, other than assaying, is there another motive for the Lord Assayer? I ask this because I feel like the intention is for the PCs to want to continue Caliban's quest to keep this island safe and secret, I think maybe it needs a slightly more pressing threat in that case.
Overall, really fun island to explore, love the creatures and the atmosphere. Fun crawl.