Well here's my take on it...I used to fish a very similar marsh. It had 5-6 FOW, mud bottom, and lots of great looking cover. The problem was, the combined issues of winter kill, low oxygen levels from decaying mud and weeds, biogas( hydrogen sulfide and carbon dioxide), too warm and stagnant water and very limited spawning areas, created an situation whereas only a limited number of fish could survive. Since this includes baitfish, that puts yet another limitation on gamefish's survival rates. Small, speedy hammer handles seem to thrive and the small largemouth become targets in place of natural baitfish.
My opinion is, your doing everything right, but the area "looks" better than it is. It's a carbon copy to the marsh I fished years ago. Have fun and hey, you never know?