I live in Mexico, so WhatsApp is a mandatory app. You simply cannot communicate with businesses or people here without it. Mexican cell phone services provide WhatsApp service without using any of your data plan or minutes for cell calls. So nobody answers a call here unless it comes over WhatsApp.
Given the amount of time I have put into getting this to work using the information provided here and elsewhere, I have come to the conclusion that it is not possible to transfer the WhatsApp chats from my iPhone to my new Pixel 7a using GrapheneOS.
I will just go through my chat history (which is extensive after living here for 3 years) and manually capture phone numbers, addresses and other data shared in the chats. Then I will just move my WhatsApp account to my new Pixel 7a phone. I will have no chat history, but at this point I just have to move on. Just not worth the effort.