I agree, if water is ok then it’s likely to be bacterial or viral. Low dose Ciprofloxacin according to @AquaBiomics’s work is definitely worth a shot IMO.I've used cipro several times with success in LPS tank. Your numbers aren't bad how about flow and lighting par?
Have you done an ICP test to check everything? Maybe something has rusted and slowly killing corals? ICP can determine stuff like that.