You have a beautiful home with scraggly trees in front of it. I don't think they will ever be truly healthy again.
But, as healthy as every thing else in the yard looks, it does not look like the soil is lacking. If one third of the root system was damaged it can't regenerate fast enough to compensate for the stress said damage put on the rest of the root system, and the tree in general. But, I think you will continue to have die back. If you prune back the dead wood you you may have a few years of decent looking trees. But, only two thirds of what it needs, so a slower decline. So, one third of the trees root system has been damaged, with two thirds still supplying the tree with nutrients. If the 'Island' is the natural contour, you have about one third of the root system out side the 'Island', and stripped down several inches. When I first started scrolling down my thought was construction damage and compaction. A grade change of just 3 to 5 inches can kill an Oak, and it often takes 5 to 10 years to completely do them in.
