No matter how you cut it your pump is grossly oversized for the pool and filter. You;re wasting a lot of $$$ in electricity. I don't think the sand is getting beat up as that takes a long time. If you take the top off the filter you should see a coating of white DE on the top of the sand with no noticeable cracks. Or, let the pump run a bit before adding DE and when you do add the DE then have someone keep an eye on the return(s). If DE shoots out, your filter is channeling.
Al