Ending with the director is actually the unhappy ending - if you consider the author's intention that is. But of course readers always choose to remain delusional regardless of obvious signs...
ending with the director would always be the happy ending. the author’s intentions only really decide whether something is canon or not. unless the author makes mc say that he’s happy the kid he basically raised killed his lover so they could end up together it’s the unhappy ending desired by the author. bad endings are written intentionally all the time.
Perhaps a case of glass half full or half empty. Up to you how you see it. In my opinion, the happy ending is Minhyung and Yijae being together because they have a lot more in common than readers care to see.
I really hope director is the endgame.. Otherwise there is NO way this has a happy ending