Sorry about your leaking pond, I share your difficulties though not nearly as bad as it used to be. The polymer treatments slowed it considerably to an acceptable rate.

Some thinking outside the box is if your dam has some good clay in it, use the top foot or two to create a seal on the bottom. Perhaps the original builder got a bit too ambitious and dug too deep. The pond may not wind up quite so deep in the end, but maybe more consistent to a reasonable level and the lowered dam will be achievable for a fill.

The rock on the bottom across such a large area is very likely to have cracks in which water can escape. Imagine a bunch of tightly packed large bricks with no mortar. Water will go where it can.