Hi,
Looks like a nice job on the setup so far.
To make the sand run smoother, make sure it is very dry. Its a good idea to spread it in the sun on a warm dry day then store it in a dry plastic pail with a tight cover. You said you tried a pressure regulator and had unsatisfactory results? My experience with using a regulator has been good, (it helps to even the flow of sand and air). Here again, using a good moisture trap in conjunction with a regulator works well. When sandblasting with high pressure (60 lbs or more, I have had to run up to 90 lbs.) the high volume of air squeezing thru the small hose will cause any moisture to condense and spit out the tip and possibly clog it.
But then again I use a 3/4" line, a 16hp compressor and a 200 gal tank, might be different.