The Notebookbar implementation Tabbed Compact is finished and can be tested. Download LibreOffice Master for

Configure Notebookbar:

Tools -> Options -> Advanced -> Enable experimental features

View -> Toolbar Layout -> Notebookbar

View -> Notebookbar -> Tabbed Compact

And this is how it will look like

The idea of the Toolbar is that you get all actions grouped in different tabs. The benefit is that there is enough space for most actions from LibreOffice and you can show action icons and labels. In addition the UI is sizable.

Download and test it.