So long as wood stays dry...
You could always dig down periodically and check the wood's moisture to make sure it's below 20%, the magic cutoff for where fungus will not tread.
Gutters with a downspout that goes a good distance from the sills could also help.
And heck, if you want to go overboard, dig a drain around the shed in the standard way:
filter fabric, 4" or thereabouts drainrock, and drainpipe draining downslope. Even that would be easier than digging out the sand.
To ease your mind: if the sand or earth touching the sill and joists is below 20% nothing will rot. I live in a rainforest and I found spruce shiplap under our cabin sitting on old forest duff. Ten feet away it's rained 80-100 inches for the last 70 years.
Enjoy your floor.
Zach