Good response. I wish I could give it two likes. Yes, Sodium is an infamous villain. I didn't know it conspires with Potassium to inhibit uptake. I keep my Potassium under 20 ppm. I have seen a chart detailing all the conflicts between micro nutrients. It's too deep in the weeds for me. But there might be one or two important interferences on that chart. As far as the Seachem Equilibrium goes, somebody posted that they use it in their low-tech tank then add a little extra Calcium and Magnesium. That makes sense to me because with low-tech you don't have to keep adding stuff.Long ago I noticed that when using high doses of potassium (about 30 ppm or more per week) with a total hardness of GH 4-5, many long-stemmed plants show signs of radiculitis of new shoots. At first I thought it was calcium or boron, but later I read articles about impaired absorption of calcium with increased doses of potassium. The best known antagonism occurs between potassium and sodium, and therefore higher doses of potassium are used for hard water (this is not always the case, hard water may not contain much sodium), but the calcium-potassium ratio has also been found to be present. As the author of the article argued, the concentration of potassium must necessarily be less than the concentration of calcium. Yes, you are absolutely correct that it is impossible to maintain precise concentrations in a dynamically changing system, but this works in the long run. I confirm this from my own experience. With a decrease in the weekly dose of potassium to 8-15 ppm, the signs of sciatica completely disappeared.
As for Seachem Equilibrium - is osmosis with a remineralizer used in non-technological tanks? I can understand if the poor quality of the water (like mine) does not allow the use of the water supply. I think low-tech tanks are made with tap water.
I hope I understood your question correctly, if not, please describe in more detail.
Rolling your own micro nutrients is technically challenging and hard to verify. It's not for the average hobbiest or the faint of heart. It's interesting that you tested for Cu. Do you test for Boron and other micros? Do you have confidence in your tests and lab work? They say you don't have to add trace elements because they come from fish food. I supplement trace elements just to be sure. You might try adding them yourself since your using RO water. It couldn't hurt. I also suspect it might have something to do with the PO4 since everything else is in order.