I’ve not yet read Peak yet but I’ve been aware of the concept of “Deliberate Practice” ever since Ericsson’s research was misrepresented in Gladwell’s “Outliers” so I have no more understanding of it than, at best, what you’ve described.
Some ideas I had around applying these concepts to less measurable concepts is mostly around the “outside of comfort zone” practice. For example, for writing, one could write outside of their usual style or content. They could write poetry, fiction vs. non-fiction, short vs. long prose, or even a movie or comic script.
Back in my high school English class, I was required to write essays to submit under different categories such as narrative (telling a story of personal experience), descriptive, expository (such as your review), and persuasive. So even under just the non-fiction prose area, there are many ways to challenge yourself.
I also try to apply it to my DJing by playing different genres, forcing new constraints, and always doing things that are past the comfort zone of, “I know how to do this really well.” This too, I think, can apply to design.