I can understand your affection for "Dark Star" Groovy Man. Many people like that song a lot. It just doesn't do it for me though. I don't know how else to put it other than to say to my ear it simply isn't as "tuneful" a melody as songs like "Helplessly Hoping," "Suite: Judy Blue Eyes" or even "Carry On" had. I sincerely hope my comments don't spoil the song for you. That's absolutely not my intent.
One other thing I meant to mention but forgot - at least as far as CSN is concerned, I think they stopped being quite as adventurous in their harmony vocal arrangements as they were in the beginning. I don't know if, even as early as the late '70s, they felt they didn't have the vocal chops they had in the late '60s-early '70s or what, but it's all been very straightforward 3-parts since then. Compare the vocals from "Helplessly Hoping" to the song Stills liked to claim was the answer to that song, "See the Changes" from 1977's CSN. The latter is a fine song, nothing earth shattering in terms of structure or melody, but certainly the equal of "HH." But the harmonies, while beautiful, are predictable and maybe even just a little boring. Whereas on "HH," if you can bring even a little bit of a fresh ear to the listening, you'll find the harmony lines are anything but predictable. They're kind of all over the map, while still being exquisitely beautiful.