When you're structuring the content, search volume seems not as important for sub-topics, as long as they answer a question, is this correct?
Yes not as important. With sub-topics on a site, you want to branch out into lucrative keywords. It all adds up. And also think about how people will come in, look at other pages / poke around on the site. Here's an example report from Keywords Everywhere of a cluster of keywords that are VERY close to a person purchasing a product.