The water iris is definitely one of the plants that my wife wants for some of our pond edges. I believe they spread very slowly.
I did have one other thought, since you have upslope silt ponds. You could plant something that spreads rapidly in those ponds in the hopes that plant takes up a significant load of the nutrients.
I suspect that would only work for smaller rain events, where the incoming water does little more than displace the existing water in your catch ponds. Also, you would definitely have to pick a plant that does not spread by seeds or broken pieces of the plant - since you don't want it in your main pond. Our plant experts might have some proper candidates.
As a bonus, a thick mat of plants in your silt ponds would more effectively catch silt and keep a bit more out of your main pond. Of course your silt ponds would then fill in even faster.
I do not know the size of your silt ponds, but renting a mini-excavator for the weekend is pretty cheap. The biggest minis have a decent reach. Further, if your pond bottoms are solid and the slope is gentle, then you can drive into shallow water and operate.
I also don't know how far you need to "throw" your excavated material to keep it above the flood line for heavy rains? If you can get by just rotating and dumping the bucket and then later spreading the spoils a little bit up-slope, then you could probably renovate your silt ponds yourself.
We've done some thinking about planting 'something' in the silt ponds, to catch nutrients, but every high water event fills them completely and then washes stuff into the main pond via the overflow ditches. As you noted, I still can't settle on a species that wouldn't take hold and maybe run wild in the main pond. I learned long ago that my 'control' over the pond is tenuous at best.
We've looked into renting a mini X and play small ball with excavating the silt ponds. My brother-in-law even volunteered to operate it (he's got lots of work experience with excavators). The Boss says I'd need to haul it 'somewhere' and not just pile it to haul later. I'd need to purchase a small dump trailer to make that work. I put that request in to The Boss, but so far it's been slid down the list below silly stuff like new windows on the house, replacement deck boarding, a swim spa for the afore mentioned Boss, and groceries. :-)
I did a bit of research on nitrogen mitigation via floating plant islands. The roots grow down through the island into the water, and periphyton grows on the roots, and then consume the nitrogen. There are even commercial places the sell them. I think maybe I could build something, but it might be a push on the cost vs just buying one? Not sure how I'd gauge success. Maybe serial water tests before and after?
