I did this on v3. Snapshot v4 is out but doesn’t have subsite backups yet.
To add Drive as a backup destination I had to create an API. It was more than just the instructions that were in Snapshot.
The tricky part was creating the Consent Page with the right settings.
I only needed the 2 Drive permissions which didn’t have the lock symbol beside them. Choosing the other Drive permissions which had a lock caused the consent page to require validation from Google, which can take days or even weeks.
Also, don’t add an app logo image because that will also require authentication.