Can't update event of non-primary calendar with service account

[Mike Halliday]

I can successfully update calendar events of the primary calendar. I can read but NOT update events of an additional calendar owned by the service event owner (response is "forbidden" with domain "global").

I'm aware of domain-wide delegation, but the owner of the service account (and calendars) is NOT a Workspace user - so domain-wide delegation doesn't seem possible.

How can I get UPDATE access to the non-primary calendar?.