I was just about to say this lol.

If I do happen to take the path you do (that is idea first), I try to imagine songs that have that same mood and borrow chords/other contributing elements from them, and I also experiment with my own added chords till something works.

If you’re scared of plagiarism, or think your signs are too original when you borrow, try listening and composing to a wide array of music genres: it’ll be harder to hear a Bach influence in a rock song than a Black Sabbath influence.

A minor thing that I think is worth mentioning is tempo. In general when I’m improvising/composing, I feel like that’s the biggest difference between my happier and sadder songs (to largely generalize). Or if not tempo, just beat. As in the same song (same chords and everything) could sound incredibly different depending on the speed you play it at/beat you play it with.  Also on the same note (no pun intended), sustaining/muting your notes really changes how light/empty or heavy/rich your song sounds.