With sardines you are of course introducing the mercury issue.
Vitamin D dosage is a surprisingly controversial topic, with many voices claiming the recommended dose is much too low and you are very unlikely to overdose if you are not taking completely unreasonable amounts of it.
If anything, you might be better with a food like Mussels/Clams/Oysters. They have the lowest levels of mercury [1], and if you're so ethically inclined, some of the lowest capacity for pain or sentience in the animal kingdom.
If I would have to choose only 1 fish to eat purely for health benefits, it would be sardines. They don't live long enough and are low enough in food chain to not accumulate much metals and other crap.
As others have pointed out, Mercury in sardines is as low as it gets.
Mercury is actually not that big a deal if you are an adult male, or a woman who is done having kids. The main issue with it is developmental issues in children.
Unrelated: IMO, Sardines are the most delicious fish.
Vitamin D dosage is a surprisingly controversial topic, with many voices claiming the recommended dose is much too low and you are very unlikely to overdose if you are not taking completely unreasonable amounts of it.