Factory resetting a GrapheneOS devices makes data on it completely disappear, as the encryption key is gone. Even if someone knew it, it wouldn't be possible.
For user profiles, you can delete the user profile and wipe the data of that profile without a factory reset being needed.
If you're using GrapheneOS, you can factory reset your device and then flash Stock OS and you should be good to go.
I recommend reading through: https://grapheneos.org/faq#encryption