Ok, I've been using Synology Hyperbackup for nearly a year now to end-to-end encrypt and backup all my personal photos to Google Drive. This is slow but a low cost way to have archival backup done.
However in the last month, suddenly the backup started to fail so I tried to do another oauth to Hyperbackup but it kept failing.
The reason for it is so stupidly trivial. I had switch the Synology DSM to map all HTTP requests to port 5000 to the secure SSL port 5001, but the problem is that the certificates will not match then and then Google Auth fails.
So you have to disable the redirection at Control Panel > Network > DSM Settings > Automatically redirect HTTP to HTTPS
And then, you can go to hyperbackup and add the Google Cloud drive. Whew, a week of confusion.
As an aside one nice feature of Hyperbackup is that you can specify a Directory and a Folder where the backup is and if you've deleted it, it will attempt to recover the backup tasks and save you lots of copy time!
One thing that I did somehow break in all the tiddling is that the Synology server no longer does a Bonjour broadcast, but it does do a DHCP client option 12 request so I can get it with Unifi as server.localdomain.
Also, make sure that you mainly use the secure SSL connection at port 5001 and not the insecure 5000 port access except for this one time case of allow oauth.