Compost as it breaks down will settle into a bit of a balance between
carbon and nitrogen. Artificially disrupting this balance will just make it tip back again (excess nitrogen leeches out, excess carbon breaks down fungally).
It sound like your problem isn't in the compost heap it is the state of your soil. I'd look to build better soil by adding lots and lots of organic matter. Focus on a small area and
mulch the surface really well (like 8 inches plus of woodchip) and gradually expand the area as you get more materials. Spread it too thinly and you dilute the effect across too large an area.
You could also try planting some deep rooted bio accumulators - these bring up nutrients from deep in your sandy soil and return them to the surface when the leaves are cut and decompose - think comfrey, plants in the thistle family (cardoons, globe artichokes). Along with nitrogen fixing legumes you could make a big improvement.
I'm not personally familiar with developing sandy soils however, so others may have more ideas.