- Enable the Calendar API in a GCP project.
- Create a service account. Download its JSON key.
- Create a calendar for bakasync alone.
- Share it with the service account e-mail. Permission: Make changes to events.
No OAuth, no browser. The service account signs its own token, so the sync can run unattended.
Step 3 is not optional advice. Point it at a calendar you also use by hand and you lose the one thing that keeps your own events safe: a calendar it fully owns.