In my opinion, and the fact that it’s just an opinion is part of the answer itself, the term stupidity or stupid doesn’t really mean much of anything. As you say it has a lot of overlap with the concept of ignorance, the main reason someone might choose to use one term over the other is to make it derogatory, or to express displeasure with a person or action. If you’re neutral about someone lacking knowledge of something, you think they’re “ignorant” if you are annoyed by this or don’t like that person, you think they’re “stupid”.

However if you look up “stupid” in the dictionary there is a bit of a difference. It focusses more on cognitive capacity than knowledge. So if you don’t know things because you’re inherently lacking in the ability to learn them effectively, people might say you’re stupid, whereas if you don’t know things because there was simply no opportunity for you ever to have learned them, then you’re ignorant.