Randomly read with focus in one hour thought it was MID, then bumped up to 4 stars. The original setup is different from the usual stuff, while not entirely Isekai, the MC is a genius of a different magic world. He and others met a Goddess who was.....traveilng worlds to find heroes? The twist is the Goddess is taking these "candidates" to forcibly merge them into a combined hero. Which allows the humans to keep and retain an unfair monopoly of power over the Demon Race. For the Demons side, they are just trying to get by as the humans are the bad guys this time? Themain plot ends a bit quick so the story is going in a different direction. MC is a skilled magical genius who did manage to make the Demon Lord stronger to accompany him on his journeys

Series wise its all right, you aren't missing anything if you don't read it