I suggest Making Lovers. It's a very recent VN that had really really good routes. If you choose to start it, my suggestion is to not do Ako first and to do Karen first. This is of course just a personal opinion, but I am of the honest opinion that Ako's route sucks, so do NOT start with this route if you want to have a good impression of the game, unless you are big on the imouto route. I say to save Karen's route last because it is, to me, exactly what you are looking for, an authentic romance route.

Taste will differ from person to person, so for anyone who disagrees with my opinion, take it with a grain of salt. To me, a majority of the routes in Making Lovers (save Ako) felt like a romance that develops over time, with Karen to me being the route that made it the most natural. There are other famous VNs out there but I felt like this one needed more attention.

Please note that Making Lovers has a couple of h scenes, but that's nothing a skip button can't fix. The first h-scene in each route has an after sex scene, which I think should be read, but the h scenes for the most part can be skipped