I'd like to propose again that a desire is not a pain, but it may produce pain or be a response to a pain.
To test a somewhat muddled analogy: fire is not a pain, but if you put your hand in it, it will lead to pain. Moving on from the analogy: if you keep your hand in the fire, you will have a desire (in this case a response to pain) to remove your hand from the fire. If someone offered you a large sum of money to keep your hand in the fire, the natural desire to remove your hand will be fighting with the (vain? depends on the circumstances) desire to get the money (choices and avoidances: which resultant pleasure/pain leads toward a better outcome?).
If desire is a pain, then per PD03 the limit of the magnitude of pleasure would include the removal of all desire. Is this what Epicurus had in mind? Then why would he describe natural and necessary desires? Does he say somewhere that gods have no desires?
Can we even experience pleasure without desire? Certainly we can by stumbling into something pleasurable. But Epicurus is very clear that prudence is of critical importance; this is how we live our lives with intention and not by chance.
As I recall from an experiment described in the book Dopamine Nation, rats with their dopamine blocked would starve to death. They weren't motivated by the pleasure of food or by the removal of the pain of hunger, but by dopamine. So if dopamine equates to desire (does it?) then it would clearly not be a pain or a pleasure. Desire would be a stimulus to action as opposed to pleasure and pain, which serve as guides to action and results of action. (OK I'm mixing modern and ancient here)