Dosing High Po4?

Low NO3 can make plants "more" red. This is a technique I stay away from.
Taking plant health to the edge by limiting a specific compound from their diet.

I've never been presented with over abundance or limitation of PO4 affecting red plants.
Many run high levels of PO4, in many aquatic environments it will curb and prevent GSA.
I target 2ppm of PO4 in the water column FWIW.

The myth of extra Fe dosing providing better red plants.
I cannot say if this is fact or fiction.
There are many views on this.

My best performance in red plants has always been the light source.

Red plants in general:
There are red plants that are red and always will be red regardless of any of the above.
Then we have many "color changelings" in the hobby also.

I hope this helps, I've answered several of your posts today that haven't seen action yet.
Maybe others will chime in and offer other or different opinions too.