That said, painful chronic conditions are something I’m still struggling to reconcile with the philosophy. I’ve heard arguments against Epicureanism that “it’s a philosophy only for healthy, happy people”. While I disagree, I’m not entirely sure how I’d respond to the criticism.
I would say that it's not just a philosophy for healthy, happy, people, it's a philosophy that is the best way for anyone in any condition to work toward the greatest health and happiness that is possible for them.
Contrary to the mystical-based Stoicism or Platonism or the rest, Epicurus doesn't offer a magic pill that death is a better place, or that every circumstance in life can be changed. There are indeed many situations where there's no further viable option to reduce pain other than to live with it and recognize that the pleasures of being alive outweigh the pains. When you get to the point that you are certain that there is no possibility of pleasure ever again outweighing pain for you in your circumstances, then and only then is it time to start thinking about "exiting the stage." But as Epicurus also said (VS38) "He is a little man in all respects who has many good reasons for quitting life."
In respect to the main question, Epicurus is reported to have said that:
"A man cannot become wise with every kind of physical constitution, nor in every nation." https://handbook.epicureanfriends.com/sbsdlx/#117
I'd be careful about how to apply that, but it only makes sense that Epicurus would recognize that everything that makes up what we think of as "us" is essentially bodily, and therefore sickness, disease, death, or the circumstances in which we live can effectively prevent us from living as we would like to live.
I'd call that realistic rather than pessimistic, and I'd call it much preferable to imagining that there is a better life after death, or a magic pill that overrides nature, becaus those things just don't exist, and I'd rather know the truth about my situation rather than spend whatever time I have under fake pretenses.