Two thoughts.

Could it be that the type of sand is not correct for pool filters, that is why the pressure readings are so odd? The other thoughts on the filter are also strong possibilities, such as too much algae, broken laterals, etc.

Would it not be easier to just drain the pool and start from scratch?

I drained our pool when we had a metal problem as it is nearly impossible to get rid of the metals through backwashing. This ended up being the least expensive solution.