I also have this issue with po4, it's usually around or above 1PPM.
I've tried rowaphos which is expensive, id throw a wholw 250 ML serving in a bag in the sump it would come down to about .6 then after a while just end up back at 1.0 or above.
I have a chaeto fuge which is growing but not growing at the rate where it can make a dent in my po4.
Also having issues with some corals receding and calcification (skeletal growth), which I think the high po4 is part of the issue.
Also following along to hear others thoughts