LPS are very adaptable. I know Cherry Corals grow the majority of their encrusting LPS on pretty low par.
I also use AFR (the mix your own powder). I am surprised your alk is above 9 considering how large the tank is and the dose volume. How often do you do water changes?
Your magnesium is low, but not melt your corals low. IO salt mixes to about 1320 magnesium or thereabouts, so when you say a steady alk and calcium, how steady are we talking?
It's hard to tell with that lighting if that's cyano for sure. I see bubbles, which can also be indicative of dinoflagellates. You mentioned Chemiclean - normally that stuff knocks cyano out fairly quickly (from what I have read here). If it's still around, are you sure it's bacteria?
I agree the Nopox may not be helping. It's hard to say for sure, but the Nopox, chemiclean, and lower lighting combined may be starving your corals.
I had good luck with the Nero 3 on a pulse mode. I made it so one goes on for 5 seconds, then the other turns on 1 second before the first one ends, etc. It gave a nice wave motion. You could try increasing your flow or upgrading pumps.
May want to consider an ICP test to rule out any heavy metal contamination.
One of the drawbacks of forums like these is a bunch of randos comes out of the woodwork with solutions. I mention everything above only for consideration, since it's impossible for me to have the full picture.
Whatever you change, it's one more change for your tank, so do one thing at a time (i.e. increase your lighting, or stop nopox, or change flow, but not all 3 simultaneously so you can identify if it's helping).