I looked up data from a calorie tracking app I used a while ago and I got just enough vitamin D. Main sources were fish, eggs and dairy products. However, I ate a lot and I'm quite sure in winter there will have been periods where I didn't get the recommend intake.

So if you live in a northern country and you don't like fish, I suppose it's quite a good idea to take a supplement.

Though as OP said, even if the sun isn't strong enough, there are plenty of benefits to going outside beside vitamin D.

Vitamin D is fat soluble, so it would be feasible to build up a supply in summer to last through winter. I've seen it theorized that a cyclical weight gain and loss tied to preindustrial seasonal food availability also plays a role.

People tend to fatten up on winter and peaking at around winter solstice

Not sure how your plan should work?!

Because of industrial food chains making that possible. Before we could refridgerate and ship food all over, most of your calories came from seasonal foods, and summer is usually more bountiful than winter.

If you gain weight during summer (so that excess vitamin D is stored in fat cells) and use body's stored fat in the winter (eat at a deficit, so that your stored fat, rich in vitamin D, is used), you need no supplementation. This is what all animals living in the region do.