However, even though I'm making quite a few changes/additions including a new chapter, it's a lot easier to patch something that exists than starting with a blank page. The fact that I don't need to churn out a huge number of words lets me focus on the new material and refactoring as needed.

When I was an industry analyst, I used to joke that the effort to write something went up by something like the square of the page count. There are counter-examples but mostly I think it's at least somewhat true given that you need to sort of keep your head around the whole project.