Congrats on your nice looking pond!

I looked at your satellite view, and thought "no way was that pond 18-20' deep". It looked like a very shallow pond in flat grass land.

However, looks were obviously deceiving me from just the satellite view. Your ground views that show the valley setting are a much better depiction of your pond.

That being said, it does not look (to me) like a pond that would have filled with 8-10' of organic muck. I suspect it lost a lot of depth due to erosion along the steep banks of the pond before they stabilized. There may have even been significant sediment filling from the valley slopes during the brief period after construction but before the good grass cover was re-established. Finally, there is probably also some sediment (and organic debris) coming from further upstream based on the looks of your valley.

Regardless, silt is much more difficult to move than muck. I highly recommend following esshup's advice with the pipe probe to try and determine how much fill is "fluffy" and how much is more solid.

I think there have been a few threads on Pond Boss where people did suck out a significant amount of fill. However, I think it failed, or was too much effort/money most of the time.

The nature of your pond construction and your geography does present another option. Especially, if your current fish population is not worth saving. You could drain the pond (fully or partially) and use a backhoe or excavator to cut a slot in the dam. Pushing silt down and out is usually much easier than trying to go up and over. It looks like you even have a spot for the muck and silt to dry out below the pond, so you could spread it a year or two later.

If that option appears viable to you, then search for some old threads on that topic, or start a new post on that specific question. Further, a completely fresh start on the fish population would probably give you a much better fishery by Year 3 with less effort and money compared to modifying your current fish population.

Finally, it does look like there are some plants in the SW corner (of the photo) on your satellite view. If not, then your old pond definitely does have something suppressing the rooted aquatic plants. It could be muskrats, carp, crayfish and/or severe competition with algae. Keep posting about how your pond progresses this spring and summer (if you don't dredge or drain this year), and the experts can give you a lot of advice on that topic.

Good luck on your good pond project!