Grab regular wheat first since it's the most reliable mill product and cooking essential, doubles the value right there. Summer and fall are perfect for growing it takes just 4 days, even faster with speed-gro. Processing beets into sugar is next best thing, getting you triple value per beet, but watch out since gold-quality beets actually lose money when milled. For unmilled rice, the value boost is pretty good and it basically waters itself near ponds, making it worth your time.

The mill works overnight and processes everything you throw in but careful not to overflow the 36 slots or you'll lose extras. Once you've got a kitchen going, that flour turns into bread for extra profit, and sugar goes into tons of recipes. Maybe interesting that you don't need to buy overpriced flour from pierre anymore, saving quite a bit especially early on.

Really though its more about having ingredients on hand than making tons of money, since other crops like ancient fruit will earn you way more. Just toss whatever you've got in before bed and collect in the morning, simple as that.