Actually, Interea is not a prequel, sequel or "bridge" between Adastra and Khemia.
It's a parallel / side / complementary story that describes in detail what happened during the remaining 4 months the main human character lived in Adastra before he had to return to Earth.
If you remember the final part of Adastra, he briefly, superficially described the trips he had with Amicus all over the moon and what he enjoyed most, which includes the remark in regards of AdRae being very much like Las Vegas. We'll likely see all that in this companion story.. or at least I hope so!!
All in all, it's a story that will mostly center on the last 4 months we (as the human char) enjoyed in full being with Amicus until we had to say goodbye for the next 8 painful years... :(