If the pool guy thinks 40 psi is OK then I'd change pool guys. The system isn't sized properly. Could be small piping, small filter, way oversized pump, or anything in the return side that is limiting the flow. I don't know anything about DE filters other than what I've learned from this forum but if we assume the filter is the restriction causing high pressure what if a grid broke and you're just bypassing the filter media? Also, what about the pressure during backwash. Did that change?
Al