I would be looking at making a dynamic form as a PDF rather than trying to do this with Word docx.

If you stay in Word there are two dodgy methods that might be workable but perhaps neither would align with your useability goals.
1. Use a Building Block Content Control to allow the user to select a chunk to add to the form.
2. Use an If field to harness the checkbox value to display or hide content. This would require a field refresh kicked off by the user to reveal or hide the content after the checkbox is changed.

If you wanted to explore the #2 option the If field would look like {If checkbox = true "Here is your content to display" ""} where checkbox is a plain text Content Control mapped to the same field value as the checkbox Content Control.

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Andrew Lockton
Chrysalis Design, Melbourne Australia