MC slowly builds a harem (basically one new girl every book) but by volume 9 it was reading like bad fanfiction.

This is worse. It's volume 7 and he has 20+ mistresses that come in all shapes and forms. I've read this volume to maybe the half point and every chapter was just some discussion about which group of girls get to fuck MC next followed by an orgy. The next chapter then starts with a recap of the orgy in some roundabout way which I can only assume the author writes while constantly giving what he thinks are suave winks. Stuff like "The last night was a war of attrition but in the end everyone won if you get what I mean...". And then it repeats with another group of girls fighting for his attention. Fanfics have more class than that.