As a "full-stack" developer who started out on the backend and took on front-end work later in my career, I used to build back-to-front. You generally end up with a cleaner, more scalable, and stable solution. That nobody wants. I've learned this lesson the hard way.

Start with the UX and work your way backwards. I don't like it from an engineering perspective, but it's the only way to go to build products.

Users just do not care what kind of horrible kludge is running behind the slick UI.