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in summary
One potential solution could be to set a default sensitivity label for your SharePoint document libraries. Any new files uploaded or existing files edited in these libraries will have the label applied if they don't already have a sensitivity label, or they have a sensitivity label but with lower priority. Make sure that you've met all the requirements for setting a default sensitivity label. You should have created and published sensitivity labels, enabled sensitivity labels for Office files in SharePoint and OneDrive, and made sure that SharePoint Information Rights Management (IRM) is not enabled for the library. Site admin permissions are needed to apply and change the sensitivity label in SharePoint.
Note that default sensitivity labels do not apply to existing files at rest in SharePoint. Also, some label configurations that apply encryption aren't suitable for SharePoint and don't support a default sensitivity label for a document library.