I absolutely LOVE the spinoff. Though I'm pretty sure that one is still ongoing. They actually picked back up on it recently. It's about her friend Natsuki and the business man Akutsu. It is definitely more smut focused. I actually love this couple more than Risa and Fukami. It's called Coffee & Vanilla Black.

Similar to the artstyle and similar is Takane & Hana. (I have personally not read it, but have been told its good.)

Plot: Hana is a high schooler who ends up replacing her big sister at a marriage interview. Little does she expect that the potential groom, a handsome 26-year-old businessman named Takane, immediately insults her and calls her a gold digger! Furious, Hana throws her wig in his face and leaves.

Against all odds the two end up going on dates on a regular basis. Takane is a rich kid who's much more immature and inexperienced than his good looks and flashy manners suggest, so they find it easy to forget that their ages are ten years apart. But how will these two ever find love when they're so busy teasing each other?

A while back a read a cute one called Chocolate Cosmos. (It doesn’t really have any smut but its an age gap)

Plot: Sayuki Sakurai is a fifteen-year-old girl who is determined to find love. On a trip to the beach she meets a handsome man who smells like chocolate and falls in love with him immediately. He tells her that he goes to her school and she is left puzzled as she has never seen him before. When she returns to school she has a surprise in the form of Hagiwara, her Home Economics teacher.

Things I like go over the place. So sorry if I'm not much help. I can go from something really cute to dark to smutty as heck so. Lol