This message is a bit late to the party but I recently successfully migrated from an iPhone to a Pixel with GrapheneOS and wanted to share how I migrated my chats.
For this I used a old Samsung Phone. With the smart switch App I migrated all of the WhatsApp chats from the iPhone to the Samsung. After that it was really easy to migrate from android to android. Just had to scan a QR-Code, wait some minutes and the process was done.
If you don't have an old Samsung phone you could also do it with a google phone or any phone with android > 12. (https://faq.whatsapp.com/1295296267926284)