I have only done one Word 2016 vba application. But mine appears to face similar challenges to yours. Since you have not enjoyed any other responses, let me share my thoughts.

Rather than mail merge, have you considered storing bookmarked documents in Word's "Building Blocks"?

My project involved three types of documents: A Partial waiver, a Final waiver and a Notary form. The partial waiver can run multiple pages. I generate a notary page after the last partial page. The one-page final waiver incorporates the notary page, so it doesn't need a separate page. And the notary page itself can be a standalone document.

I used bookmarks in the documents to mark for the variables and saved each document as a Building Block entry in Word's Quick Parts (Insert > Text > Quick Parts > Save Selection to Quick Parts Gallery ...).

Loading and populating the forms through Word VBA forms is not particularly intuitive. And I overstayed my welcome with my posts to the StackOverflow forum as I struggled with it.

But Building Blocks might be an alternative to consider.