One way I have gotten the phosphates down on several instances was to get 1 or 2 oz of Phosban in the tank changing it weekly with 15% water changes using RO
(the deionization part is important for this) water with a new membrane and in 3 or 4 weeks it should be down to zero. If not, then your problem should definitely be on the feeding side. All this, while pruning all the hairy stuff you can.
I don't think LR
will leach PO4s...
I once heard a guy who had a problem with its aragonite sand he used. He got it from some pet place and did not rinse the sand well and he had the problem for over a year and after hundreds of dollars spent in Phosban... He said there was always a milky silt in the sand when he vacumed it. He supposed that might've been the problem.