I'm assuming you're referring to this page https://faq.whatsapp.com/1295296267926284
Based on WhatsApp's documentation they allow transfers using only this one method. Ideally, WhatsApp would add more ways to transfer data from one phone to another, but that's up to them.
Unfortunately, I think the only way you can do this is by using Google's proprietary software. If I were in your situation, I'd do the following:
- Back up your iPhone data, just in case.
- Either find anther up to date Android phone or go back to Stock temporarily.
- When setting up the new user profile on Stock, transfer data from the old phone following WhatsApp's instructions.
- After the data has been transferred to the Android phone, follow these directions to back up WhatsApp data to Google Drive.
- Flash GrapheneOS again or switch back to the GrapheneOS phone and install the three Google Mobile Services apps
- Restore the backed up data in Google Drive to the GrapheneOS phone following Whatsapp's instructions. Others here on the forum have had issues with this. You can refer to this thread https://discuss.grapheneos.org/d/422-how-to-restore-a-whatsapp-backup/13
I know that's a lot of work, but it sounds like this data is very important to you otherwise I wouldn't suggest going through all of this trouble.
Also, a little disclaimer. I've never tried to transfer WhatsApp data in any way. I don't even use WhatsApp. Theoretically, my suggested method should work, but I've never tested it, nor have I read about others on here who've done the same thing. As I mentioned in step number 6, others have had issues restoring from a Google Drive backup. You could also run into other unforeseen issues that people on here might not be able to help you with. Ultimately, this is a WhatsApp restriction.
If you do attempt this, I wish you luck!