in my own perception,
the cause its no parasites or infection but rather environmental. im 99% sure.
i experience these before during my hypo as well.
to simply say, its like HLLE to me. as you observe closely on the picture of your sailfin tang, the dots are more of aligned with the lateral lines on the side of the fish and the finnage is uneven to me.
looking at your YT also reminds me of my YT during hypo. "dots on the head"
main cause includes not enough "elements" in environment causing those HLLE, another contributing factor might be pH too.
and a factor thats contribute to HLLE is ozone, but ur ozone dosage is fairly low but I will suggest shutting it off completely till your fish recover or else the finnage will be hard to recover or perhaps never going to recover from my experience with higher dose of ozone.
if you are worried about pH, use more "complete buffer". baking soda is not a complete buffer but it is still useful and cheap at times. and even if you use baking soda, theres a fear you might overdose. so take note.
this problem can be counter off with you starting to increase your salinity back to standards. though they are feeding, it is not enough. dosing amino and vits to your food and water is a plus^3 point. trace elements too. so take note.
and discard your smelly rotten egg water outside the tank, when its done with the smell, then drip back into the tank.
do something before its too late to reverse it.
all the best Joe