When you add the sodium hexametaphosphate, the bits that cloud the water are calcium phosphate, which gets filter back out. So you add PO4 in the SHMP, cloud the pool with CaHPO4, using H+ and Ca++ from the pool, and leaving the Na+ behind, then in the filter you attach Cu+ to the CaHPO3, possibly as CuCl2, and backwash it away.

So some of the phosphates you add as SHMP may end up leaving as a calcium phosphate + copper dust.

I'd recommend taking care of the copper first. Once the copper is to an acceptable level, you'll probably need to do a lime-softening run to lower your calcium somewhat. That will likely remove some phosphate, too. If you can aluminum sulfate available, you can remove more, using that. Once you've gotten the copper gone, the calcium to a manageable level, and then reduced the excess alkalinity, you can check and see if the REMAINING phosphate is at a level of concern.