I also have extreme sensitivity to Sunlight for over 30 years, with my version of POIS.
I was diagnosed by a consultant dermatologist maybe 20 yrs back, with "Photo" aggravated seborhaeic dermatitis. Basically, a few seconds exposure to direct sun on affected areas causes a flare up. Creams and avoidance of sun has helped a lot over the years, but I avoid direct and bright sunlight with determination, as it has a very negative effect on my mood also (extremely so during 3/4 day POIS hangover).
I also underwent a series of light de-sensitisation treatments about 15 years back (under the care of same consultant) which improved things a bit more. Basically, I had to take some (special) prescribed tablets and then stand in a machine once per week for 4 wks or so, to get exposed to certain UV lights to then desensitise me. It did a help a good bit also, but I still have to avoid direct sun and over wear sun glasses. A pain in the ass for my family and for me (wearing baseball caps always in Summer) and as I loved the sun as a kid.