I started on that in the the prior post but deleted it because i was getting too long.
In summary, my view of that is that Epicurus is speaking conceptually as to the nature of the goal.
The essential premise that is unstated here but which is present in PD03 and recorded at length by Cicero in Torquatus' explanation of Epicurean ethics, there are only two feelings, pleasure and pain, and when you don't have pain you have pleasure. All this is reinforced immediately in the letter by the following:
Quote129] And for this cause we call pleasure the beginning and end of the blessed life. For we recognize pleasure as the first good innate in us, and from pleasure we begin every act of choice and avoidance, and to pleasure we return again, using the feeling as the standard by which we judge every good. And since pleasure is the first good and natural to us, for this very reason we do not choose every pleasure, but sometimes we pass over many pleasures, when greater discomfort accrues to us as the result of them: and similarly we think many pains better than pleasures, since a greater pleasure comes to us when we have endured pains for a long time. Every pleasure then because of its natural kinship to us is good, yet not every pleasure is to be chosen: even as every pain also is an evil, yet not all are always of a nature to be avoided.
It's my view that Epicurus expected Menoeceus or any other student of his reading the letter to understand the wider context of there being only two feelings. Unfortunately, people today don't know the context, so they don't understand how saying "absence of pain" is the equivalent of saying "pleasure."
To me that context resolves the part of the passage that is arguably the worst if not understood. "When we are pained because of the absence of pleasure, then, and then only, do we feel the need of pleasure."
When you know that "absence of pleasure" means "pain" just like "absence of pain" means pleasure, you realize that he's just stating the obvious. When you have pain, that means there's a gap in your goal of life that needs to be filled with pleasure. When you have no pain, that means there is no gap in your goal of life, because your life is already filled with pleasure. At that point, you have no further need for more pleasure, because your life is already filled with pleasure. This does not deprecate pleasure but emphasizes that pleasure is the only thing that really counts.
You'll also see here on the forum lots of discussion of "the limit of pleasure" as referenced in PD03 and in reference to an argument by Plato, who alleged that because pleasure has no limit then it cannot be the ultimate good.
By showing that pleasure does have a limit, and thus there is a state which can be described as "the best," Epicurus refute's Plato's argument. When you think about it, it's obvious that the best state of pleasure is 100% pleasure. At that point, you have no need, or ability to experience, anything more.
I have several collections of quotes on this as I'm constantly working on a better presentation of them. Here are three:
A second is here: https://wiki.epicureanfriends.com/doku.php/the_norm_is_pleasure_too
And a third:
A Gate To Be Burst: "Absence of Pain"
I can't be sure which gates Lucretius had in mind, but I can suggest one "gate" that is particularly in need of bursting, as it stands directly in the way of a better understanding…