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yourmother Realistically, there's not going to be much difference in the metadata you send to WhatsApp using GrapheneOS compared to stock Android. There are some benefits of the additional hardening of GrapheneOS, as per the features https://grapheneos.org/features listed, but most of the metadata is provided by you using the app itself.
GrapheneOS does help limit WhatsApp's use of sensitive permissions, which can help somewhat. The camera, microphone and location permissions you can give access one time only, if required at all. Only select Files and Contacts can be shared with Storage and Contact scopes features.
On WhatsApp, conversations are end-to-end encrypted, meaning the content of the messages can't be read by WhatsApp. But WhatsApp lacks a feature like Sealed Sender from Signal which when enabled has minimal metadata collection: https://signal.org/blog/sealed-sender/. Broadly speaking, this means that WhatsApp knows when and with whom you are speaking, but not what you are saying.
Metadata can be collected and used, as per WhatsApp's privacy policy: https://www.whatsapp.com/legal/privacy-policy.
Their privacy policy can also be arbitrarily changed, the most recent of which in 2021 generated a fair amount of controversy:
https://www.wired.co.uk/article/whatsapp-instagram-facebook-data.
Hope that somewhat helps to clarify things for you.