I would do it over from scratch
Take it from experience
If you have the equipment it's only going to cost you time and fuel
Definitely put a proper core in and compact bottom and sides

With my pond after a whole host of possible solutions ( re- coring the dam in the front, re- coring the dam in the back, adding benotinite then polymer You name it I've done it. i'm not sure which one helped the most but it seems to be holding up now

Someone suggested something interesting to me, and in hind sight it makes sense
Everyone makes the dam side the deepest part of the pond ....that's where you are most susceptible to leaks and failure. He said that I should have made the opposite side deeper and the dam side shallow

You definitely are on the right track asking questions here
I'm sure you will get the correct answers

Best of luck