A friend and I write together, via email, back and forth a little at a time. we have already once taken a few hundred thousand word story and painstakingly copied and pasted it into a Google document.
First off: STOP! Ha—don't take it the wrong way, but it has to be asked; why are you eschewing direct use of what you know will be your eventual destination, Google Docs? Docs works basically everywhere now. So suggestion 1 is; give the idea of collaborating, or taking turns, right in a Google Docs file serious consideration.
If sticking with email, I recommend you install a full-fledged email application on a computer (so; Windows, macOS, or Linux), connect it to your email account, and then export the relevant emails from there into a format Docs can import, like plain text or PDF. After import, it would need some grooming, but I suppose you're accustomed to that already.
- you can use this extension and export it into Google Sheets / Excel / CSV
- free version is limited to exporting 50 emails per month
- paid version costs from €24.99 ($28.37)
- quick tutorial on how to export it: https://youtu.be/hqgXcxIuJRA