i write music a lot, but I normally start with the instrumental and do melody/lyrics later, but i noticed while browsing through questions that you start with melody/lyrics first and then the harmony comes to you later.
that's mostly not true. the harmony is usually 80% baked into any melody. it's kind of like how most houses come with furniture, and even if you change the furniture it's still gonna be pretty close, like you're not gonna have the refrigerater in the tub, except maybe once in a while