In a filter that size, it wouldn't be too expensive to give Zeosand a try.
We don't recommend zeo sand. More here, zeo sand vs silica sand
Some filters actually recommend that so might be an option.
I had the same thing happen to me a couple years ago - and as a last resort - I added this to sand in the sand filter - it seemed to solve the problem. I vacuumed the sand at the bottom of the pool and it didn't come out the return.
Like you - I followed all the instructions - and I used the quikrete pool sand.
Amazon.com : Purifiber Cellulose Fiber Swimming Pool Filter Media D.E. Alternative - 15oz (0.94lb) : Swimming Pool Chemicals And Supplies : Patio, Lawn Garden
I have same pump and never had the issue.
My guess: Either the pump is defective or the fact that your return is lower than the pump (maybe creating some siphon effect) is causing the issue.
Maybe dig down so your pump sits lower than the return?
Here's something to think about I guess- my laterals clearly had sand inside them. That can only be either laterals that are out of spec (gaps too wide) or sand to small. Am I right? Is there larger sand or some other material?