Importing a Microsoft Word document as a FrameMaker Book (not document)

I've just upgraded to 2019 from 2012. I work in an environment where

1. An author writes the first several drafts of a fiction manuscript in FrameMaker.
2. Once done, the FM document is ported over into Microsoft Word so that the editing team and beta readers can collaborate during this process.
3. Once this process is complete, the document is ported back in FM for typesetting and publishing.

We're dealing with fiction manuscripts holding close to 300,000 words each. These are non-XML documents.

In Microsoft Word, each chapter lives in its own section. We've upgraded to 2019 to leverage the import function that would allow us to bring a document from Word into Framemaker. From the preliminary exploration, the expanded import feature maps Word paragraph and character styles to FrameMaker's which is great. However, I do not believe that this import function can map each chapter in Word to a document included in the book.

The alternatives would be to import the word document into FrameMaker and

1. Keep it as a single document. Would there be performance concerns with FM 19 x64 with documents holding 300,000 words?
2. Turn the document into a book manually.
3. Rely on a script to do that. This would be preferable and before we sit down and write such a script, I wanted to ask if you would happen to know of such a script?

FM 19 has gone a long way to simplify the interaction between FM and Microsoft Word, especially since it is able to publish HMTL which Word can import. Turning a document into a book would be a great addition.

Thanks for any help or feedback you might have on this.