How to manage large fillable form documents?

> Now imagine a 50 page document with 50 or so fields on each page, and you find one mistake you have to correct, or you decide you have to add two pages somewhere in the middle. You will then have to go back to the application that you used to add the fields, and redo the entire process.

This workflow is not necessary. You can generate a new static file and then use the Replace Pages command in Acrobat to replace the pages of the old file with those from the new ones. All fields you added to the old file will remain in place. Of course if the layout changed then you'll need to adjust their locations, but you won't need to start all over again.

Another option is to use a script to add the fields, in which case you can do it with 100% accuracy, as opposed to the unreliable (and often unpredictable) fields detection wizard command in Acrobat. A script can identify certain characters and add fields on top of them.

For example, I've developed two (paid-for) tools that can create text fields from underlines and check-boxes or radio-buttons from specific Wingdings characters, with 100% accuracy. You can find them here:

https://www.try67.com/tool/acrobat-reader-convert-underscores-to-text-fields

https://www.try67.com/tool/acrobat-reader-convert-wingdings-characters-to-check-boxes-or-radio-buttons

@helpful_dancer157F if I am understanding the main question, can you create a fillable form that was created from a Microsoft Word 50-page document?

Yes. With the use of Acrobat Pro. Under All Tools - Click on Prepare Form. And it will create a form with text field properties. 

I went ahead and double-click on a text field to open the properties, and I can edit the appearance of the font among other things.