Thank you for this guide. I had a similar situation (old Samsung with normal Android to new Pixel with GrapheneOS) but it didn't work by just copying the files in /Android/media/com.whatsapp/. It didn't offer me to restore the files. I suspected this is because it didn't know how to decrypt them but no idea what was the real reason.
In the end I was able to use the built-in transfer from one to another Android phone. This is a Whatsapp feature but you need to have two phones so it wouldn't have worked in your case.
I also wonder if it would be possible to do a Google Drive backup (I think you can enable end-to-end encryption in the settings) on the old phone, install Play Services on GrapheneOS and download the backup from Google Drive, and then stop any further cloud backups and remove Play Services again. As Play Services are a normal, sandboxed app like any other on GrapheneOS, there should be no trace of the Google account left on the phone afterwards, right?
