I’m going into college with a declared major.
Sadly, I don’t have a lot of classes related to this major, and currently none of my essays (even supplemental) show why I want to go into my intended field. Should I scrap one of my supplementals and change it into an essay that explains my interest?
You don’t give enough info but I think the need for HS experience in one’s intended major is overblown except perhaps in a few fields such as computer science. If you want to do engineering, you need to take calculus and physics if your school offers it most won’t have access to engineering ECs in HS.
Many top schools don’t even admit by major.
Do you mean that the college you are applying to admits by major or just that you know what you intend to declare? Some state publics are by major and some aren’t. Some top privates do and some don’t.