I'm looking for manga, where the mc is already in a relationship or where they start going out early on. However, I'm not looking for Slice of Life series or (only) romance drama. The manga should have a preferably serious plot besides their relationship and neither character should die early on.

Examples I can think of are Haru no Houtai Shoujo (for an established couple) and Bokura no Kiseki (for an early established couple).

Hoshi no Samidare
Magico

DONDAKEE!!

Ah right, Hoshi no Samidare kinda fits as well. Will look into Magico, thanks!