Yep, it's a Herculean task to communicate all the subtleties. i'm fairly comfortable with "happy" in the sense of the Declaration of Independence referring to the "pursuit of happiness" as if that word somehow embodies all the attributes of the best life. But you're right that the way it's interpreted today is much more fleeting.
I do think that a large part of the problem is that it probably also implies more than any single feeling, even "subjective well being." When I see how the translators are using that word to express what you're talking about in terms of blessedness, I don't know that any word or term that focuses primarily on any sort of limited experiences is good enough. I'm thinking more in terms of that Sedley article which compares Cyreniac to Epicurean happiness and talks about how the Epicurean view was more of a total evaluation than a temporary feeling. It also implies something that we'd likely consider to be "objective" in the sense that we can all understand and communicate that this is fundamentally the #1 goal of life. Calling it "subjective" is certainly true in a sense, but it probably implies in English that we are very narrowly saying that we ourselves completely define what it is. Yes we do in a way, but the 'feeling of pleasure' that plays such a large role is given to all of us by nature, and there are "limits and boundaries" within which it operates. If there weren't, we'd never even be able to explain to each other what pleasure means.
In the end maybe I'd equate this to Torquatus saying that Epicurus held "pleasure" to be the highest good. We're talking about an evaluation of a full life, and I suppose that's necessarily an abstraction.
Unless and until we can communicate the seriousness and importance of the ultimate goal, how can we hope to begin to connect with the seriousness that comes through in Lucretius' poem and Epicurus' own work.
I think your comment hits hard on one of the big tasks facing us. We speak English and we have to convey accurately in English what the pursuit of Epicurean philosophy -- and of life -- is all about.