If your Gmail-as-sender setup uses a personal Workspace account (not a service account), Google's token policy expires the refresh token at 7 days unless your OAuth consent screen is verified.
Google's OAuth 2.0 spec says refresh tokens are perpetual. Their actual implementation expires them at exactly 7 days when the consent screen is in testing status. If you launched your sender setup with a personal Workspace user + an unverified consent screen, you'll see your sequences silently fail every Friday.
Book a 15-minute scoping call and we'll tell you whether your case fits the standard pattern or needs a custom approach.