Having done a lot of work in both spaces, trust me -- there's nothing luxury about getting this work done in the public space.

Also, extensively planning and specifying requirements ahead of time is exactly the opposite of how you pull off delivering well-designed public UX like this.

In the public space you inherently never have a single person who can call all the shots, because organizationally you're accountable to all of the public. Orgs have to be set up to carry on operations resiliently outside having a controlled lineage of management. How you do things is far more directly and heavily influenced by legislation, and that landscape is constantly shifting over the course of your project.

BY FAR the private sector is where engineers can enjoy far more luxury.