Well it's half a sequel and half a spin-off.
You do not play as the Human, so it is not a direct sequel and COULD be considered a spin-off. Thing is, the events taking place in Khemia are following the events taking place in Adastra (with a 2.5 years gap, but remember the last line of Adatrsa (spoiler): Scipio walking in the Throne room and warning Amicus about the Diplomacy Mission. What does Khemia begin with? Scipio going on the Diplomacy Mission).
Keep in mind however that both Interea (a companion novel to Adastra (spoiler) focusing on the Moon Tour the Human and Amicus go on after Cato's death (in Adastra, these events are merely briefed)) and Khemia are still under heavy development. Therefore they're short. Khemia's on hold since almost a year now; due to a writer's block and because the writer wants to focus on Interea rn
Hope I helped, have a good one mate