I’d suggest reading this series of blog posts on Vitamin D by Dr. Deva Boone (they were also shared here before):

https://www.devaboone.com/

If you’re deficient or not getting enough exposure to the sun around midday, then supplementing could be useful. As a preventative measure for COVID-19 or as a measure to make COVID-19 less severe (or for reducing the risk of any other ailment), the blog posts cited above conclude that there is no evidence. There are correlations, but we know that correlation does not necessarily mean causation.

Please read the blog posts and arrive at your own conclusion.

Not an expert, but as usual, it depends. You may want to check your levels changing too because you can overdo it https://en.wikipedia.org/wiki/Vitamin_D_toxicity Talk to your doctor.

IANAD, but reasonable algorithm looks like:

If you can test, test. Look online what normal levels should be - you'll find several options, pick the higher ones (look for Vitamin D Council for an upper bound recommendation)

If you can't test or you test below optimum and you can sunbathe - do that. A lot. It's inherently safe as far as overdose goes. For skin cancer, it's usually worth the risk, and definitely worth it during the pandemic (unless you have specific reasons to think you're higher risk).

If you can't test and can't sunbathe... in normal times I'd suggest to supplement except in summer. But in Covid times, I'd go with light supplementation during summer (<1000 UI daily) and higher dosage staring autumn (~2000 UI).

Before starting supplementation you should check with your doctor and he would have known from your last annual exam if your vitamin D was low. If you get plenty of sunlight then you most likely don't. If you're a vampire like me then you absolutely need supplementation (my doctor prescribed it about 5 years ago when I showed very low blood levels of vite D).

Not an expert at all but moderate dosage should have no side effects. Regular Overdose by 2000-3000% can have some long term negative effects.