Honestly, the best way to end a series with a harem is the way Jitsu Wa Watashi Wa ended. 196 chapters, and MC starts dating one of them at around chapter 60. The rest is their life until graduation. Eventually, most of the other girls in the harem start meeting other people and falling in love, so everyone is happy in the end. The MC grows a whole lot too and he isn't dense in the least. It is the female lead who is the dense one.
It is honestly one of the best manga I have ever read, not just for rom coms. Great humor and art (once the author gets into his paces). He does the best expressions. Plus, every side character, no matter how trivial, ends up being important.