As you make the decision to choose developmental editing services, I encourage you to look up some resources about developmental editing and book publishing. Some that I have found most helpful include those from the site of my teacher, Dr. Laura Portwood-Stacer:

https://lauraportwoodstacer.com/publications

Another good resources is the book, Developmental Editing, by Scott Norton, available through the University of Chicago Press:

https://press.uchicago.edu/Misc/Chicago/595146.html