I do think that a siphon drain at the other end, with the pond being further deepened at the new inlet, would get sediment/muck to slide towards the new drain. It may take time ( years?) but it probably is the best option for a regular maintenance feature. Perhaps with aeration stirring the water, it may work fast.
I am not sure about cleaning up those edges. They are steep, and more digging at the edge will probably get the ground to collapse into the water. You sure don't have the leeway to lose more ground there.
