I used a Maddox/madd-ax all my life, it is not a shovel, not a hoe or an ax, but it can do the job they do in skilled hands.
It is best for breaking up that first layer of red clay, under the four foot of sand you have already removed.
When an large animal died on the farm, we dug a 6' X 6' X 6' deep hole so the wild animals & plow would not unearth the animal.
I never knew why 6 foot cube was the right formula, I was just glad it was not an eight foot cube.
Lucky you, I am guessing if I dug down 3 feet I might hit solid rock.