I like the compost pit idea, though in some areas, the paver on top of the pit would need to be
big.
I would, first and foremost, attend to the water retention issue with bentonite clay, or natural clay, silts, whatever you can get your hands on that is a different particulate size and composition than what you have, preferably smaller, as indicated.
I would follow the suggestions of heavy compost additions, and again, whatever you can get your hands on that is natural and in a state of natural, healthy decomposition. This will not only import organic matter, which you need desperately, but also the healthy decompositional organisms on that organic matter. Duff from the forest floor is ideal, if small amounts are harvested over a large area.
Also, if you wanted to accelerate things, go get some red wigglers, and instead of compost pits, alter the composition of the pit contents to fit worms; instant outdoor vermiculture. I put some in my composter, which never got hot enough to properly hot compost, but does amazingly well as an outdoor vermiculture bin, and in select spots in my
garden bed where I had mulched with mostly finished compost. The difference was astonishing. I suddenly had seeds sprouting that had remained dormant in the spring, and you could track the worms' regular traffic by the change in the texture of the soil.
If you wanted to do something on top of the sand in the meantime, I like those reuseable felted fabric pots. I have made shallow rectangular fabric boxes out of landscape fabric and planted into those while I remediated soil that needed more work than early season prep.
I have also seen people buy bags of triple mix, lay them overtop of the future garden area, and simply plant directly into them, either making small holes for individual plants or cutting open the upper side of the bag to expose soil.
I would be much more sanguine with this approach if the bags were fabric or paper, or some kind of garden-compostable vegetable plastic, but by putting holes right through these "instant garden beds" into the soil-to-be underneath, the action of the roots of the garden plants puts organic matter down into the sand, the action of water carries soil particles into the sand, and the sand is covered, allowing any water to stay longer, helping to foster soil life longer in what is usually a very dessicated environment, thereby speeding the formation of soil.
I accidentally used this method when I had rhubarb poke up through the last, unused bag of topsoil in my garden. I was careful to remove the plastic at the end of the season. It's certainly possible, and I noticed no scraps of plastic contaminating the soil in subsequent years, though caution is necessary.
Hope some of these ideas are helpful. Keep us posted, and good luck.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein