Sand? Pure sand? Forest duff is better. Sorry, all soil is great soil the only caveat is that one learns how to manage that soil. Us humans thinking we can make soil better has been one of our worst downfalls in horticulture.
Three feet of forest duff? That says an awful lot about your environment. I made vegetable gardens in caliche clay just by dumping decomposed organic matter on the surface and a preliminary double dig to make a 'raised' bed that would never need any more 'tilling' but most certainly will need to be watered and fertilized.
The sand is a done deal, your rhododendron will just have to make do. Rhododendrons evolved with clay soils, partial shade. Slow growth to allow the forest debris to decompose enough that there might be a bit of chemistry to add for the plant in addition to the great tilth of the soil.
Your Rhody needs a balanced fertilizer in the worst way. Regular water. Sand, sigh, why do we humans think we know stuff about drainage, plants, soil ecosystems when we don't? Careful with mulching. Their roots are very shallow and adding more than 1/2 inch might smother the roots and their access to oxygen.
So they told you that they could not BUILD structurally on the forest floor because it is too thick and compressable? Wow. Compression is necessary for building structures. The duff is not compressable. It is pure undecomposed organic matter that is very very flammable. A fire can be started by a spark that will smolder for months with no signs of smoke before it bursts into fire. I actually tested this and by golly, no smoke at all. Looking closely into the duff you most certainly can see embers, hot red burning stuff with no smoke!
Duff is amazing and I would have removed the duff and then double dig to fluff up the soil the original indigenous soil, then plant and add a bit of balanced fertilizer. Decomposed organic matter thinly on top of shallow rooted plants. This is acid loving so I would do a pH test for sure.
Plants are amazing that they can survive what we humans throw at them. Chop them completely down, plant them in sand and hope for the best? And look at this guy! He is mightily trying to survive with NO chemistry, no comfy clay and organics, sitting out there in bright hot sunlight surrounded by hot gravel. This is a forest floor plant. Partial shade, cool soils, clay soils SOME chemistry with which to do photosynthesis is all they ask and this plant has none.
And I see signs that glyphosate was sprayed in the last month. Is this true?