Lots of good suggestions here.

It kind of depends on the size of your document and the content. Adding 25 large separate chunks one at a time can be tedious too. Another approach, and one I find easier in really big report documents, is to have all the sections in there, and just remove the ones not needed, ideally by doing so in Navigation Pane. For other documents, building step by step makes more sense.

The Content Controls are going to be the easiest to, say, repeat a client's name or a title all the way through. If users are not adept at changing the content controls, I found this video to be interesting as a way to do a little hand-holding. You'd make a user form where the user would fill out a bunch of details upon opening the document, and those entries change the contents of the Content Controls, which populate through the document. https://www.youtube.com/watch?v=8lKzMuk2nO0&t=59s

You can also use bookmark/cross reference, but I find that some of my coworkers are really good at NOT typing within the brackets of the bookmark, or backspace deleting them, thus breaking it.

Also if you already have a bunch of insertable detail in an Excel spreadsheet, a mail merge can also work. You may think of mail merge as merely working to create form letters, but it can be used to transfer a lot of database info into a single Word document. A coworker prefers to collect all her info in an Excel sheet, and we set up a merge that imports all that data into a report section.