I'm using Seachem Flourish Phosphorus and testing every few days to maintain PO4.
I began with the "feed more" approach as well, but my Ostreopsis got much worse. It wasn't until I tested that I noticed I had high NO3 (25 ppm) and very low PO4 (near zero). Since dosing PO4, no Ostreopsis mucus strings are visible in the display. They used to show up minutes after lights went on, and now they aren't even visible after 10 hours of light.
Hope this helps as it seems you may be experiencing a very similar scenario.
This definitely helps, I appreciate it. How much are you dosing with the seachem and how often are you testing for P04? Also what exports do you use? I only have cheato and a skimmer