What is the thesis of the book?

Instead of opposites, I would consider both Workaholism and Procrastination as forms of experiential avoidance coping strategies. Instead of a golden-mean between the two, the ideal point is off the line, where you have the psychological flexibility to live a valued life despite difficult feelings and avoid both traps i.e. Acceptance and Commitment theory.

n.b. I see you are using ideas from Tim Urban, I would suggest referencing him properly rather than just copying.