This shouldn’t/ won’t be a quick process, otherwise you will seriously stress and/or kill your sps. I had a similar situation with a mature tank, it took over 6 months of GFO and 10% weekly water changes to get it down and my PO4levels were half of yours... with high levels like you Have GFO can be spent in 24hrs and will need changing or it will leech PO4 back into your system... LaCl is another possibility... but the precipitate needs to be removed somehow... I used about 25Kg of GFO to get my levels down so it’s not going to be cheap...