It's worth remembering that vitamin D blocks the production of melatonin for a few hours, so it's better to be taking it in the morning (assuming you're not a shift worker).

I have a relative in the PNW who tells me that he had had SAD every winter but changing all non-bedroom illumination to 6500K ("daylight") has helped a lot.

That's just one person, but worth trying out as medium-base LED bulbs are under $5 these days - even the high-CRI ones (look for 95 or better).

That said, I've seen evidence that light therapy is effective for many things. MedCram on YouTube has a number of videos covering light as therapy. Francisco Gonzalez-Lima has used trans-cranial near infrared light to boost impaired cortical activity. He’s hypothesized that photons are acting as ‘stand ins’ for electron donors in the electron transport chain (critical for cellular energy production). He’s done similar work using methylene blue as a replacement ECT donor making up for cytochrome C oxidase down-regulation.