I consider there being no point in supplementing D3 unless you take K2 at the same time, and there are plenty of experts who say that almost all of those living in northern climes need some level of vit D supplementation. The benefits of sufficient Vit D3 are huge. Bone health, immune system, sunburn resistance, dental health, lifting depression, and vit K is usually deficient in the western diet, with implications for calcification of arteries, bone, teeth and heart health.
Well worth reading up on both and if you decide to supplement then make sure you are buying decent quality and taking adequate amounts. Personally, I consider generic chainstore multivits and mins inadequate and a waste of money.
I can’t recall seeing anything about vit D/K benefiting insulin resistance, but I’ll go out on a limb

and say that expecting a body to function at its best while experiencing chronic nutritional deficiency, is asking too much of it.