Larry Versaw wrote:And the weeds are not having any trouble growing either
You mention the sunflowers, tomatoes, and corn is all doing well. However, I see similar yellowing and spots on both the sunflower and the tomato. One of the tomatoes appears to have blossom end rot (though the picture is not good). Yellowing of the leaves is almost always a nutrition deficiency. However, the cause could be the nutrients aren't there, or there is not enough water to make it available, or there are pests stealing the water or nutrients.
I'm not an expert, just trying to throw out ideas to see if something matches or gets the experts thinking. I would check water. Dig down a few inches or try the chopstick method. Check all the leaves at different times of day to rule out pests. Do you ever move your plants around? Could this be a disease built up in the soil? You mention the last couple years have been the worst. It definitely sounds like something is out of whack and the plants can't get the nutrients or maybe there is disease built up.
For the future, do you have the space to try a garden in a different spot? Maybe till some manure into this garden, plant clover or something and let it sit for a year?