According to the Society for Editors and Proofreaders, the aim of copyediting is to "ensure that whatever appears in public is accurate, easy to follow, fit for purpose and free of error, omission, inconsistency, and repetition. This process picks up embarrassing mistakes, ambiguities and anomalies, alerts the client to possible legal problems and analyses the document structure for the typesetter/designer." Copyediting works for your project if you have a mostly finished piece of content and you want it polished and checked for errors.
The Editorial Freelancer Association describes developmental editing as, "developing a manuscript into a coherent, readable work." Developmental editing includes big picture feedback such as how chapters or sections flow together, whether the goals of the manuscript are met, and recommended additions or cuts. This process does not include final copyediting.
This type of editing includes the copyediting process and may also include, "extensive rewriting or restructuring." This is a great fit for you if you have readability or audience concerns or if you want extensive feedback.