Thanks
Based on this, and also borrowing from the list of sub-genres on the Kobo and Kindle stores, I've made a few so far: military sci-fi, space opera, alien invasion, post-apocalypse/dystopian.
I also have categories for artificial intelligence, first contact, and colonization, though I've found there's a bit more overlap with these ones.
Also, I'm not sure if 'first contact' should be expanded to 'alien contact' — I checked Amazon for examples and it was mostly romances where the so-called 'alien' looks suspiciously like a human, lol.