Given the time span of this problem a two year old ICP test is quite recent and these specific RTN symptoms were the same then as they are now. Still I have another ICP test planned soon. I've tried so many things over so many years there is hardly anything left to try out.
I really wanted to spare you the huge wall of text that people would not bother to read anyway, but nobody today at least has encountered RTN in this way so here are some quick answers.
For maintenance I've left it alone for 6 months to extreme care and everything between.
I use test kits for 7 parameters and measure for stray voltage and check for corrosion.
For water changes I've tried from none to 15% a week for months.
I've tried acros mostly in the past, but now I aim mostly for the easier corals like turbinaria or montipora.
Yesterdays parameters: Ca 420, Alk, 8.5, Mg 1450, Phosphate 0.02 Nitrate 0.20.
For flow right now I'm using 2 Tunze Streams and the return pump of course.
I've tried lots of different flow setups using 3-6 pumps.
For lighting I have 3 Coral Care and two Red sea reefer led lights.
I know of tanks of similar size that can keep the most light demanding coral alive with 3 Coral Cares.
Of course I've tried lots of various lenghts and intensities and even placed same species high and low for comparison.
Hammers have done well and I had lots of Monti caps since they grew very well, but I lost them all over a few months time about two years ago.
The fish are doing grea tand I have no problem keeping fish like mandarin, anthias and copperband alive for years.
You totally forgot to ask me about biome and bacteria since in the modern reefkeeping world of boat rock and limited bio diversity that is where I'm looking right now for answers. I had success with SPS for the first 10 years in the hobby, when I knew a lot less than I do now.
I have 20+ years of experience, abundance of patience, heaps of knowledge and the will to carry on.
Enough of text already.