If you have achieved the most efficient possible cost of production, then you may be wasting your time farming as well, unless you also are a maxed out gatherer.

If you are maxed out on both, then you probably still are giving up a little bit of profit by not focusing on one or the other, depending which you can do more efficiently in terms of units per hour /played.

Keep in mind that your profit on selling food is the selling price minus what you could have sold the mats for (i.e., your income from production minus the opportunity cost of selling the mats directly). You're not making more profit because you're farming, and you're not changing how the supply chain works. You're just another farmer, who happens to have a single customer.

You might be a good farmer or you might not be. Others may be more efficient at it than you are, in which case you're probably leaving a bit of gold on the table by not focusing on the thing you're better at. But regardless, when it comes to selling food, your competition is other food makers, which includes all of them, whether they farm or buy their mats.