Humanoid in shape and size?  I'd like more elaboration on their design.

If you need help upping their alienness, one way is to give them a distinct psychological trait from humans along with some distinct means of reproduction (mother nature on earth has quite the play book for this).  Think of them as animals in a habitat and how their social behavior is at their most primitive.  Than you arm them with larger populations, civilization and carbyne infused bones.

Due to sharing a world, expect some carbyne to end up in use by Liotrent's bugs at times.

Chances are they'll have more than one culture, so defining their behavioral differences on culture alone isn't enough.

Though I will say I like the idea of some species that organizes in insular city-states with geothermal tech.  Do they also have tribal groups outside the city-states?

Either way I like the direction you are going with this so far.