Koi to Uso, Domestic na Kanojo Recommendations
Both adult theme related, talking about relationship hardships and more real world scenarios than other anime out there; in my opinion koi to uso being the better of the two
Both are about complex situations like where someone loves someone, who loves someone else, who loves the first person or that kind of business.
Both have love triangles.
Both have mc with darkish hair and already in love with one girl.
In both shows, the two girls/love rivals have a close relationship.
In both the anime, mc is very passionate about something.
The mc has a special bond with girl he is not in love with in both the shows.