Try spot feeding as a thicker paste with a syringe or bulb feeder rather than broadcast feeding. I was just reading that reef roids have tiny particle size food for goniopora, so maybe you don’t have enough coral to consume the small size before degrading.
I also read some food is preserved with poly-phosphate and any unused food will contribute to nitrate and phosphate as you know. We change up what we feed each day, so maybe that’s an idea for you too. Mysis shrimp, rotifers, pellets on rotation and then coral frenzy for my target fed goniopora and gorgonians.
As I research more on the phosphate balance, I’ll share with you. I’m not running GFO, but I do use phosgard routinely. I just saw something last night that says I shouldn’t be using phosgard more than a week at a time due to something about metals, so that’s important to check.