I liked "Seveneves" too, and I'd make all the same observations, centrally the one about "geekporn"--rarely do you read so much exposition in a novel, and rarely do you enjoy it as much as when Stephenson does it. This to me is classic science fiction, in which it's the science itself that makes the story so enjoyable.

I know what you mean about the vast difference between the first and second eras of the story. Not sure he had enough material in either one to flesh out an entire novel, though. Or at least the first half wallowing in orbital mechanics; perhaps in the second half he's built himself a world that will now provide the setting for a series of stories. That world has possibilities, not least because it bears some resemblance to post-singularity universes like Charles Stross's Saturn's Children or everything by Alistair Reynolds.

You're not done yet, but it's not really a spoiler to observe that for me it seemed to end abruptly with no particular climax (or perhaps I mean it didn't really seem to end at all), but I took it as a sign that Stephenson would be returning to this universe to continue the story. Enjoy.