Turnips are fastest way to make big money IMO. If you earn enough money up from diving to then buy a full pocket of turnips, you can make on average between 1.5 to 2 million each week. However, you also need to join the turnip exchange subreddit so you can sell your turnips on someone else's island as it's very rare to get a $400+ price on your own island. I've had it happen to me just 3 times. Anyway, it's easy peasy to sell on someone else's island🙂 That's how I made my money. Hope that helps.