Without draining completely and taking an excavator to dig out all the accumulated organic matter, I am afraid you will be fighting the FA for years to come. You could try the products that are on the market to sequester the excess nutrients, or just put that money towards cleaning out the pond. Renting a 2" or 3" trash pump and pumping it as dry as possible, then digging out the organics is the way I'd go if it were my pond.
There are places here that will rent "long stick" excavators that people use to clean out ditches. Maybe only 1/2 to 1 yard bucket, but they have them here that have up to a 72' boom, so a long stick could get to the whole pond as it tracks around the shoreline. Typically the pond will be drained down by a trash pump, a sump dug in the pond, then the pump placed in the sump to keep the water from puddling up in the rest of the pond.
You say 100-200 gpm. inflow from the Spring. That's 6,000-12,000 gph. From the pictures I find that hard to believe, but in any case, a 3" trash pump will pump 14,000-15,000 gph, a 4" trash pump will move around 24,000 gph, which is more than enough to keep the pond dry as it's cleaned out. If it pumps out way more water than comes in, it can be throttled back to pump less gph. You don't want to keep pumping out ALL the water and loosing prime on the pump. That would be a PITA to deal with on a constant basis.
In looking at your pictures, I would recommend that you start controlling the cattails that are growing there now. Aquatic rated glyphosate with surfactant applied according to the label. If you let the cattails go, they will completely take over the pond and you won't be able to access the pond in a few years. Now if you will go the drain/excavate and refill route, then don't kill them because they will be removed when you remove the built up sediment.
Yes, it's not as cheap as just treating the FA and cattails. But at some point in time the aggravation of constantly having to battle the FA will cause you to clean out the pond, and all the $$ spent managing the FA will have been thrown away.
Before you do anything though, I would use the boat and a piece of EMT tubing to try and measure the remaining muck depth in the pond, and see if you can get a clean water sample of the incoming spring water to test it for nutrients. knowing those two things will help you to make an educated decision on what route to take to save the most $$ in the future.
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