Coral Island

Good news this one seems a LITTLE bugged in our favor. This unlocked for me after I had bought everything except for the last 6 back items available in the clothing store at A village rank. I had also purchased everything available at the beach clean up event from the puffer fish store. I had NOT bought the golden chicken outfit or anything from the monkey vendor. I had completed every museum collection and offering at the temple so I had every available. In order to buy all this you'll need around 2 mill shells so easiest to do in an afternoon once you're deep in the endgame after getting all of the money related achievements.
Unfortunately you REALLY have to do this the long way on console by clicking 1 item then buy then repeat which I found to be a pretty insulting slog tbh but whatever. We're all trapped on Coral Island for a long time not a good time. But at least this one unlocks good luck out there.