There is a minimum effort workaround.
Name your Manuscript/Draft folder in each book (typically with the book’s title).
Using the Compile Format Designer settings you have for your projects, you’d want to retain them in your combined project.
So, back up a project. (key)
Then, rename the projects, for example, to Compilation. Strip it of all documents (send the lot to the trash and empty it). Since you can’t delete Research, trash its content. The Manuscript and Trash folders cannot be deleted either.
Open each individual project in turn and drag its Manuscript folder, which you’ve given a name, as a sub-document of Compilation’s Manuscript. To limit editing to one source, always edit the original book; trash it in Compilation when you need to replace it, then drag in the latest update.
It’s now, you’ll run into a problem, because Book 1 runs for 104 chapters (as an example), so Book 2 will kick off with chapter 105. You’ll pick up the same problem if you have parts.
To overcome this, call your Chapter Header in Book 1’s Compile Format Designer “Chapter Header1”, for example, and “Chapter Header2” in Book 2’s Compile Format Designer, and so on. You’d need to do the same with the Section Type in each book and assign them to the respective Section Layout name. These are one-off transactions and are perpetuated throughout the project. Key to this is having the same headers, Chapter Header1 and Chapter Header2 (yes, repeats of the same design) in the Compile Format Designer of your Compilation project. That way, when you drag in the respective books, they’ll find their unique Section Type links to the Section Layouts and kick off with unique numbering for each book. (You may have to assign them each time in the Combination project when they’re replaced, not sure. But then, drag in the new edit before trashing the old should resolve that.) Do the same for parts.
Any other decorations can be left untouched in the Compilation project.
So, once set up, it’s a matter of editing each book in the original project, trashing it in Compilation, dragging in from the original book project, and compiling on the Compilation project.
You could probably sort out everything I’ve suggested in an hour, while trashing old edits in the Compilation project and replacing them each time would take less than a minute per project.