In addition to the cover crop ideas, you might look into taking a Haney soil test or the Illinois N test to see how much organic N and/or protein-amino-N you have in your system. With that much organic matter, you might just need the right biology and stimulants to kick things into gear. I’ve induced excellent N cycling in vegetable crops by using IMO #4 or #5, and homemade protozoa teas, when I had good organic matter and good levels of bacteria and fungi, even if they were “sleepy” due to high P and K from high compost applications.