It would still be great to know conceptually why water changes do not seem to impact phosphate at all in some cases. I had the exact same experience in my first nano tank, where 50% water changes would hardly impact phosphate parameters. Since then, I blame the precipitations from Kalkwasser dripping after 18 months, but this might be as well just a mindstory.
Short term fixes such as GFO or LC seem problematic to me. GFO will strip out all traces according to Oceamo testing, and LC works great but if (accidentlally) overdosed might impact fish/tang health. And it would be a perennial thing to do.
Accepting the actual (high) phosphate measurement as suggested seems like a better approach. However, not being able to control any continuous trend except fixing the symptoms vs the root cause worries me as a newbie.
And all of my thoughts probably do not make sense as they might miss the big picture, but this was my thinking here, as I would like to avoid this specifc issue with my new tank if possible at all.