Who claims that high PO4 locks up Zn in aquatic plants?
I think it's due to more growth in your results.
If you are limited by one nutrient, say PO4, adding say 2-3ppm will switch the most limiting factor to something else. This has NOTHING to do with binding of anyone nutrient with another.
It has to do with Liebig's Law of the Minimum.
If the focus is on all the nutrients and providing non limiting ppm's/amounts, then you do not chase one low nutrient level to the next.
This "chase effect" causes many myths in aquariums.
Many added PO4 and end up with algae, many did not. So we know it must be some other cause than PO4. If you add lower CO2, but have a system that is more strongly limited by PO4 than say CO2, adding PO4 will dramatically increase the rates of growth, then the demand for CO2 will go up dramatically.
So now you are CO2 limited, whereas prior, the limitation was PO4. Indirectly, ou provided stable low CO2 to the plants by strong PO4 limitation. PO4 was much more a bottle neck than CO2.
When you test PO4 independent of CO2, or Zn or which ever other nutrient you chose, now we see that there is no effect.
This means that Zn or the CO2 in the above example was not independent, so you cannot say say PO4 additions cause algae, nor that PO4 binds Zn, that's not been demonstrated not matter how hard the folks on line want to yell and shout.
Here's how you know:
I add plenty of PO4 and Zn, much like you did.
If it was due to binding as the myth/claims state, then adding more Zn/PO4 would increase the binding effects, more concentration.
At least that's what we might expect.
I have no issues with my plants at 5-10ppm of PO4. No issues with Zn either.
I add a fair amount of each.
I'd stick with your plan, you might consider tweaking the CO2 a bit when you add PO4 and add more traces in general, least you fine another trace metal you might be lacking later.
More light= more CO2= more nutrients.
Stick with that.
If you want less nutrients/growth etc, use less light= less CO2 = less nutrients.
Fairly simple idea and common sense.