RootPlease honestly, I don't think that you'd be able to restore the files even with root access. The biggest issue isn't that you don't have root access to the device's storage, it's that the file was stored encrypted on the drive. If I understand the following from the website correctly, each file has its own encryption key.
File data is encrypted with AES-256-XTS and file names with AES-256-CTS. A unique key is derived using HKDF-SHA512 for each regular file, directory and symbolic link from the per-profile encryption keys, or the global encryption key for non-sensitive data stored outside of profiles.
Assuming you installed GrapheneOS correctly, obtaining root would be next to impossible without lots of money. There's an easy way to do it, but that would mean factory resetting the phone first.
So, considering all of this, if you have any chance of recovering the files, you'd need help from an expert, and even then I don't think it would be likely they'd be able to help you. Or there may be another way to recover files that I'm not aware of, but the method you're suggesting doesn't sound possible.
Sorry for being the bearer of bad news.