You nailed it: Eternum's MC lacks personality, and that's actually the biggest flaw of the game. It is a low effort, self-insert protagonist that you find in hundreds of harems and Isekai. In contrast, the plot is fantastic. That's why is really a pity we don't have a better MC. I actually appreciate harems with well written MC's, but there are really, really few of them.
My favorite harem MC is Yamato, from Majikoi, a japanese VN. Great story and hilarious. The story is set in modern Japan but with a lot of Edo period, samurai stuff. The protagonist has great personality, a cynical asshole at first, but he grows a lot. I really recommend!
But other than that, I rate Eternum quite high in my list.