[BrainlingPdx]

Not trying to be mean, but you really don't make any sort of case for doing this here. You just note you did it because you've become more comfortable with the front end. It also seems like you're designing and implementing an entire system by yourself in which case you can do whatever you want. When dealing with a team, and an API layer that has to be used by more than just your client, I don't see any compelling case presented here to start designing API's specifically for UX use cases.

[DEADB17]

The techniques are not mutually exclusive, it depends on the context. When you don't have a clear idea of the requirements starting "front to back" helps to understand and gather them while building something that validates them. For your example "back to front" sounds like a better fit.