Your fear of the repair being the weak spot is a valid concern but if the repair is done properly it should blend into the existing dam over time. Using the proper material to make the patch and packing it well will be necessary. Refacing the whole dam with clay is a good beginning to seal the patch and having it blend in well.

A few years back we had a dam breach in a flash flood. The breach was large enoiugh to drive a semi truck through it and was 15 to 20 feet deep. Clay is at a premium in our area but the dam builder that did the repair managed to find enough to mix with the existing soil along with a good quantity of bentonite. The whole mixture was packed in with a power tamper in small layers. It was labor intensive but it paid off. The repair is not leaking at this point in time and the pond is looking good. We placed a drain gate and 3 foot down tube, to replace the original 12 inch tube, in the breach while we were making the repair.

The price sounds reasonable. Good luck on a successful repair.

Larry