There are 2 other VN tied to this.
One is Khemia that takes place about 2.5 years after the end of Adastra with Amicus and Neferu going to Khemia to continue the alliance. The author released the first 2 chapters of this story but was having some writers block so he put it in hold and instead is working on the other VN .
Interea take place during the 9 months the MC spends on Adastra before returning home and is all about the tour of the moon that the MC and Amicus did. It actually helps with the Khemia story by providing background regarding characters and history that Khemia has to deal with but would have required alot of flashbacks. The author has only released the first chapter of Interea so far, but the next part should be coming out soon.
Once Interea is done, the author is going back to work on Khemia.