For some reason author had to turn Suimei into a harem protag, including the denseness. Which i think might be a result of editor meddling? I feel like that would make sense considering the way his character changed slightly around volume 3 when he ended up having more than 1 girl on his party. It also ruined the awesome dynamic he had with Lefille during the second volume as well. Her character was never the same after that point.
That aside, one big deal about the light novels is that they have some of the best fight scenes i've seen in any media ever, and while Suimei starts OP compared to the vast majority of the inhabitants in that world, he pretty much doesn't grow any stronger as of volume 8, and the threats he faces are even stronger than him, to the point where he just barely avoids the worst case scenarios, which emphasises the gap in power between the forces of evil and the forces of good (against whom he ends up fighting in more than one occasion). So in a sense they handled the "OP main character" trope surprisingly well, since being too strong or not is relative to the threats he faces.