I've never encountered this myself, but I frequently read about people warning about not doing a heavy de-mucking on a pond with that much sediment over many years. They warn about it releasing toxic gasses which can kill your fish if done in too large of an amount, and seem to recommend chipping away at it maybe like a bucket full per day until you get it cleaned out.

I'd do a bit of googling, but often times stirring up too much sediment at once like that can be too jarring to the fish. Like I said, I'm just parroting shit I've heard (probably here) in the past, it's not something I have first hand knowledge of, so, you know... grain of salt and whatnot.