Adding metadata to document libraries: custom content type or default Document content type?

For your goal of consistent metadata across multiple sites, a custom Company Document content type is the better starting point. Create it as a child of Document, rather than modifying the built-in Document type.

In SharePoint Online, use the SharePoint admin center’s Content type gallery to define and publish it across sites. Add the shared metadata as reusable site columns, then add the published content type to the relevant libraries. Creating separate, identically named content types in each site won’t give you central management.

A practical split is:

• Shared metadata such as Department or Document Category belongs in the content type.
• Library-specific metadata can remain as columns added to that library.

The benefit is consistent definitions and more manageable updates. The trade-off is that published changes need time to propagate and should be tested before broad rollout, especially when making fields required.

Also plan for existing files: adding Company Document won’t automatically convert documents already using Document or populate their metadata. Make it the default for new uploads, then update existing files in manageable batches.