Most fish provide at most a few hundred IUs at reasonable intakes, plus since both fish and dairy are protein sources I imagine the more dairy you have the less fish you'd eat.
Giraffe, I'm not sure how representative your study is since it contradicts FAO data showing Spain consumes considerably less milk than scandinavian countries. Overall people eat far less dairy the closer you get to the equator although there might be isolated countries that buck the trend. The prevalence of lactase persistence also increases sharply the further north you go.
That people in the North dont have lower circulating D levels might be due to genetic differences or the sparing effect of higher calcium consumption, but that Vitamin D production from sun exposure is lower in the north is I think indisputable, no?
Lots of people report having issues with even moderate amounts of supplemental Vit D so likely there can be subclinical disturbances in calcium.
I live in Northern Europe with even further North heritage on one side of my family and I've had similar thoughts to what you're proposing.
Beyond it being abject common sense I personally have really ****88 my health up with higher amounts of vitamin D.
I don't do well on above 1kiu and I probably need even less than that.
As long as you have antioxidant status, I believe the sun can be used as an efficient and potent source of energy and relaxation, meaning less calories are needed and giving the liver a much needed rejuvenation period without the stress caused by unintuitive fasting.
I think sunshine as enjoyed/craved is key to human health but I don't think vit D is a big part of it.