Seems like your light and CO2 are in good shape, so a different deficiency seems likely.
I don't see any micros, other than iron, being dosed. Are you dosing them and, if so, what are you dosing, quantity and how often? The discrete brown dots almost look like a severe zinc deficiency (K seems good) and there may be other micro deficiencies involved. Some of the older leaf yellowing may be a Mg shortage. Do you know what you Mg levels are? Are you dosing Mg? Lastly, although I don't think it's a PO4 deficiency, if it were me I would get PO4 up into the 3ppm area for high-tech.
At some point, you may want to take a look at the roots and make sure that they are white and not browning. What is your substrate and how old is it?