The only thing that has been kind of inferred in this conversation, but I haven’t seen explicitly stated: it’s my thought that the void/fruitless/vain desires are those that are infinite and therefore unquenchable. These are desires that are by definition impossible to satisfy, because of the lack of any limit.
This is exactly where the limit of pleasure conversation comes in. Pleasure can’t be the goal itself because it is unlimited. Until Epicurus asserted that there IS a limit to pleasure, and it’s the point where you have no pain in the mind or body.
I am in general agreement with the recent line of discussions about pleasures / desires that have no limit.
But I don't think we have a complete picture until we address the question of "how much" and/or "how" to pursue those pleasures / desires that are potentially unlimited.
Let's take "power" for example. That's pretty unlimited in the abstract, and the power to rule the world on its face would be very very difficult to justify as a goal. But the power to exclude burglars or murderers from your house is highly desirable. It would be very easy to start playing word games with these issues, so the questions need to be more clear, and I am not sure it is possible to get much more specific than Torquatus did when he stated "the principle of the classification" being that some categories are more difficult to obtain than others.
Once we identify the relative difficulties, I doubt we can get much more specific as to a "rule" to apply given individual differences in preferences and circumstances.