DeletedUser122 When I delete files on GrapheneOS, are those files permanently deleted, or is there a chance they can still be recovered?
No, they are not permanently deleted. Files on GrapheneOS can be fully recovered using advanced forensics until you delete the user profile the file was stored in. Limited information about the former presence of the file can be recovered until you factory reset your phone.
DeletedUser122 Does it make sense to think about deleting unused space?
Deleting unused space is not an effective data erasure procedure for flash based storage such as the one in your phone. Profile deletion and factory reset will wipe the encryption keys in a secure way, such that the files are cryptographically shredded and can never be restored.
DeletedUser122 Should I use a program like eraser on USB drives that don’t have encryption and a password (If a person use them to backup content from grapheneOS phones) if the threat is the government?
There are no known effective way to erase a USB stick. Always use strong full disk encryption when storing things on USB sticks. Older USB sticks could sometimes be erased by overwriting the raw storage twice, so if you have already stored sensitive data unencrypted in the past, try that. But it didn't work for all USB sticks and is unlikely to be effective for newer USB sticks.