As you've shown, you cannot do this.

You should follow your ideal path and follow @linda-lawton-daimto advice.

Workspace APIs including Slides are not governed by Cloud IAM and so setting the Editor role on the Service Account is ineffectual. Workspace APIs work with OAuth scopes which you're correctly setting.

The problem is that Workspace APIs protect user content and the only way an arbitrary Service Account can access Workspace API user-content is:

Domain-wide delegation to approve the Service Account for this purpose Designating a specific user for the DWD'd Service Account to impersonate. There's a hacky (!?) alternative approach which involves sharing a Workspace document (e.g. Slides presentation) with an arbitrary Service Account's email address (~ {account}@{project}.iam.gserviceaccount.com).