The Answer: Use "Add Cloud Storage" to turn one Files Tab into a 1-stop shop for accessing those other libraries!
Each link (seen below) will open that library in Teams (that's right, it's better than some bookmark that opens a new window), even if that library doesn't exist in Teams (ie, even if it's just some normal SharePoint Online library with no associated MS Team):
Working with the files: Even better, I can still edit, co-author and chat with my team members on a document that lives outside Teams:
Channel 'Navigation': If you happen to be someone that likes to see 'buttons' for all your libraries listed down the left-hand side, you might consider making a Channel for each library. The Channel's only purpose would be to give you access to click on a link that opens that particular file library. For example:
Search: From a Search perspective, one downside to this approach is that my Teams search box is only going to find documents that exist in Teams that I'm a part of. I could still, at least, search my "NotTeams" documents by hitting the "Open in SharePoint" button and using the "Search this library" search box.
For organizations just starting with Teams, wondering how to incorporate all their existing pre-Teams SharePoint Online libraries, this could be the answer!