The most successful APIs I've worked with have been done this way. In one project we were rebuilding an old outdated UI. We started with being very strict abou

The most successful APIs I've worked with have been done this way. In one project we were rebuilding an old outdated UI. We started with being very strict about literally only implementing what was needed for frontend, for example not even a GET /widgets/{id} until there was a UI that needed it. We also explicitly built new models for this, separate from our database models, so it wasn't just "return everything in the table": if the UI only displayed half the fields, that's what was returned. A big benefit of this mindset was when we needed a list of things from joining multiple tables, there was no thought of pushing this to the front-end; we just made a viewmodel, did the work server-side and returned it. Also, pretty early on we implemented Swagger (OpenAPI) and built in docs to our code. This helped establish it as a part of dev and the coding style that any new devs quickly bought into, as opposed to an afterthought everyone sees as a chore. After we had a a dozen or so APIs done, we started opening it to customers. We'd add new things they'd request (but our UI didn't need), but strictly kept the use-case-first mindset, as well as continuing the UI port (page-by-page, effectively). Even a few years later, the feedback from our developer users was overwhelmingly positive, along the lines of "this is the best API I've ever used".