Cleaning out pond muck is not my area of expertise, but I would think that leaf/organic muck would be much less dense and compacted than silt infill brought in during high water events.

If my "assumption" is correct, then your material may be amenable to removal by pumping. Do you have a good place to pump the sloppy mess?

Do you have a trash pump? You might be able to perform a test by running your suction hose down in the muck without your intake screen and see what you can pump onto the bank.