This item is still in the works.
Also, thank you for the additional feedback. I will circle back to the feature team and provide a new update soon.
Current SharePoint Document Library Tab only supports adding the root folder of the SharePoint library. Most teams/users need a way to get to specific folders on other SharePoint Sites/Libraries and having a Tab from within Teams will be great way to enable this capability.
This feature is on the backlog. I will update you when we have more details to share.