I think you are on the right track, but I would love to see more responses on this, as it’s an interesting question.
This offers a bit more discussion on the issue: catholicharboroffaithandmorals.com/Unchaste%20Thoughts%20and%20Desires.html
If I’m understanding correctly, a voluntary impure thought would be where you willfully allow a thought into your mind so that you can enjoy that thought and the pleasure the thought brings you. An impure desire would be to actually will to move beyond that thought and go do that thing.
An example might be if a man sees a woman and finds her attractive, so he willfully imagines what it would be like to have relations with her. However, he doesn’t actually want to do this – he just wants the pleasure the idea gives him. Given the opportunity to do this, he would say no, but he likes enjoying the idea. This would be an impure thought, if I understand correctly.
An example of impure desire would be if the man were to think, “Wow, I’d really like to have relations with her.” He actually wants to do that thing. He might decide that there is no way to make it happen or that the consequences would be too great, but his desire is to commit the sin.
Now, if the man, upon first seeing this woman, has a thought pop into his head but upon recognizing the impurity of the thought rejects it and does his best to push the thought out of his head, he has not committed a sin at all, because he has not willfully engaged in the thought.
Of course, it’s not hard to see that a voluntary impure thought could quickly lead to an impure desire.
Normally I try to post a response unless I’m sure I’m right. I’m posting here even though I might be off track so that I can be corrected if I’m wrong.