A significant portion of CAS's fiction has cosmic elements, but I really think that the poetry is where he expresses this theme most deeply. That said, I feel that "The Seven Geases" is a good allegory, among the tales, of "cosmic indifferentism".
Within the poetry itself, I would focus if I were you on the material contained in the volume
The Star-Treader. In my view, CAS in his writing career veered away from cosmicism
per se, at least to a degree, and towards a more general interest in the weird, the exotic, the fantastic, and the decadent. Perhaps Sterling's arguments against over-pursuing the subject of the stars registered with CAS, or perhaps he simply felt (as Sterling seems to have felt, regarding his own work) that he had exhausted the theme.
For more general thoughts on cosmicism in this forum, and a mention of other writers whose work, or aspects thereof, reflects cosmic themes, you might want to read
this thread, as well.