Mechanically quite deep. There's a ton of systems and they seem to slot together in a way that makes intuitive sense. I don't mind the slow burn, but a lot of people probably will. Particularly with such a weak hook. Deliver a letter so you can leave an RPG maker island is a bit weak and not a great motivation to overcome such a low power start. Even stuff like stardew has the immediate intrinsic hook of turning your shithole into a nice farm, which you don't have plans to abandon eventually unlike in this game. Though this might be mitigated later when the placeholder assets get replace and the island is more appealing to explore, or when the introduction section is more fleshed out.
You have a solid foundation. The presentation just needs some serious iterating on.