This could be done without programming by simply using an IncludeText field for each report. I don't think I can describe it, though.

Make sure your reports are created using the same template and formatted with a designated heading style for the title. Use other heading styles appropriately for sub-headings. Each of your topics is also marked with a Bookmark (name) in the template.

The reports would be named DeptA, DeptB, DeptC, etc. and put in the same folder with your overall report. You would use the IncludeText field to incorporate their contents. You would be including DeptA#Topic1, DeptB#Topic2, etc. then going to Topic1

When the new reports come in, copy them, with those names into the folder. Open your overall report document and save it with a name that reflects the date. Press Ctrl-A to select the entire document and press F9 to update the fields. Resave the document. Press Ctrl-A again and press Ctrl-6 to unlink the fields. Resave the document.

This assumes that the only fields that are important in the document are the IncludeText fields.