Two approaches to clarify an approach to the ultimate goal come to mind offhand:
1) Is the healthiest functioning of an organism stress free? Not entirely. Being stress free means that one's needs are met, but stress indicates a need for change and is a healthy response to stimuli. As humans, we can arguably be happier and more productive if we subject ourselves to a certain amount of stress (exercise, thinking about the value of serenity &c). But certain baseline mental stresses (fear of gods, fear of death &c) work against healthy functioning; eliminating these produces a baseline of serenity that allows for healthy functioning regarding other stressors.
2) Looking at the desires: to my understanding the necessary desires are pretty much a given that are specific to each individual at a specific time and are relatively easy to satisfy to maximize the specific individual's pleasure. The unnatural desires are, also, pretty much a given to each individual at a specific time and need to be fled from to maximize the individual's pleasure. This leaves the natural and unnecessary desires (Emily Austin's extravagant desires) as a potentially huge group of possibilities for pleasure, and the most active field of choices and avoidances (or engaging and fleeing). If we were to do nothing but seek serenity and avoid pain, then this category would be nonsensical.