I don't really follow what you are saying.
If the tank is nitrate or phosphate limited (which I don't believe is true for phosphate, anyway, but lets go with it), then the amount of iron is not important. 'Limiting" means it is the nutrient whose value is critical for growth. Usually, that is only one of nitrogen, phosphate, light, iron, other trace metals, etc.
At 0.04 ppm phosphate, I would not think that is limiting growth of most organisms.
I'm not sure why you think iron would be "causing" bacteria to flourish to the point they cause STN. Bacteria in our tanks are not, as best I can tell, usually iron limited (except perhaps cyanobacteria in some cases). I've never heard of a bacterial bloom from adding iron, while other things cause them, indicating those other things (such as organic carbon) are usually limiting.
Why do you think reducing the three part is useful? I don't recall seeing any values for calcium and alkalinity. I'd check those before altering dosing of them.