Ancient Fruit wine with the artisan perk. Mass produce ancient fruit and fill a few sheds with kegs. If you have the island id recommend using it to grow starfruit, its more valuable than ancient fruit when put into a keg, but also more labor intensive.

If you are looking for passive ways to make money you could use the island to produce fairy rose honey, each worth about 1000g each harvested every 4 days. You could also fill a shed with crystallariums and mass produce diamonds.

I do all of the above along with a few of all available animals, all of which gets turned into cheese/cloth/mayonnaise/truffle oil which benefits from the artisan park along with the wine/jelly. All in all I pull down about 7-8 million per month.