Dear Sbellmore,
Thank you for posting in Microsoft Community.
Based on the description, you want to link SharePoint Excel file to automatically fil/update local Excel file. To understand your requirement correctly, could you please provide a simple flow chart of the requirement.
SharePoint Online Library store files and can be open using Excel application and sync two ways in real time. We can also use OneDrive sync client to sync local copy and online copy.
For more information, you can refer to this article Sync SharePoint files and folders (microsoft.com).
We look forward to your response. Thanks for your cooperation. Stay safe and healthy always.
Sincerely,
Zee | Microsoft Community Moderator