I think you have a contaminant. After reading through, my suspects would be the metal from the old styro - or the glue that attached it; or possible use of inappropriate silicone from GC.
I had glue from partical board that wreaked havoc on my 180. I was using old cabinet doors as a lid over my sol blues for keeping scrapers, toothbrush, tongs, etc. When I placed tools on top and water ran over the edge, it would bring some of the glue into the tank. Entire colonies died overnight - mature ones, some that I'd had over 10 years - and it seemed random - SPS here, a chalice there over a 6 month period. I understand that pressure treated lumber is now treated with copper - used to be arsenic. If there is anywhere that water is coming in contact with the wood and getting back into the system, that could be the source of your problem. Same would likely hold true for Kilz primer or any paints with antifouling agents.
If it is metal toxicity, it could be absorbed in the rock and leaching out. That would explain why you have better success following a large water change and then slowly slide back into the funk. I would keep a fresh poly filter in there at all times. If it sinks, it likely has absorbed metal. I have used them to remove large quantities of copper following a heater fire. At low levels of metal, they will absorb it and sink without necessarily changing color. I'm not sure what color aluminum would turn them. (presumably what was on the styro)
Glass Cages primarily makes reptile enclosures. It is entirely possible that your tank was built with GE2 instead of GE1 silicone. GE2 has antifouling chems. And unfortunately, I don't think that there would be a way to tell other than asking the company if they ever vary in which silicone they use on different tanks. You could have been built by a noob at the company that didn't know the difference. Eh, tank looks good. Ship it. Let's hope that's not the issue as it would be a terminal flaw.
Lights: I think we all have issues dialing in new LED's. I sure did with my sols. I also run ulns but without all the gadgets. Corals were most unhappy with me following the change. If the Vegas are anything like the Sols, 30% is way too low for SPS - and most anything besides chalices, acans and shrooms. Most people seem to run over 70% (for the Sols) when growing SPS. It is possible that you are now starving corals - between ulns and low light. Even if you have corrected the contaminant, starvation would still look like the same issues. That's the pattern I fell into until I increased my lighting.
Good luck. Lastly, another poster mentioned that you are changing too many variables at once. Adding and deleting reactors can cause issues that keep you from finding the one true issue. You may solve the problem but never know what it really was.
Honestly, it sounds most like metal contamination to me. As a service company owner, I've seen many forms of it - brass fittings in plumbing, my own heater fire, pennies, corrosion falling into the tank, pumps with metal in contact with water...
