I was looking at your pictures and it looks very much like how mine started, an old pond, stagnant, full of muck, debris, but trees all around so hard to get equipment in.

I very much recommend a plan to deepen as much as you can on the first go around. If I had known what I know now, I would have had pumps on hand to pump empty, better equipment (long arm excavator) to get the bottom deeper etc. Since I have a ground water pond with not enough clay to seal on its own, I have to deal with water table changes, sand veins etc. The plan at the time was use a short arm excavator from sides, and bulldozer to push it out of the middle. But it wasn't dry enough for that plan and as they punctured the water vein the pumps weren't keeping up and the dozer had to quit or get stuck.

My backup was to run a water line from the well and fill it frequently. The soilfloc has at least kept my filling from the well a minimum and the periodic heavy rains help keep it much fuller than before.

Only by mucking out the years of muck/leaves do you get a good headstart in reducing nutrient load, and also for the first time you will truly know what the soil is like at the bottom of the pond. If it is gravel, sand, you know then you might not win with soilfloc and you might need a supplemental source for water (even a stab well / shallow well near the pond) If you find mostly clay and you plan for it, you might be able to use the clay that you find, recompact it correctly in the bottom and sides, and when it refills, you won't even need soilfloc.