The problem is that you're probably chasing ghosts.
TBH testing RO never occurred to me, I’ve always just looked at TDS: whether the output is 0 or thereabouts. If not I look at the inline TDS meter:both sides of the membrane.
Phosphate in the parts per billion does not say anything about the state of your RO filters. It might tell you something about your DI resin since that's where the phosphate is remediated, but even then it's not an accurate indicator of filter health. The only final test that's really worth anything is the TDS. It's difficult or impossible to test anything else in deionized water.