The planned look is refreshing and indeed modern.
Pinned Notebook Mode
In pinned Notebooks mode, I would like to see the Notebooks removed from the endless vertical list and individually accessed as with the Windows 10 version, listing only their own Sections and Section Groups alone.
Horizontal Tabs
I have use cases for both horizontal tabs and vertical tabs (in pinned Notebook mode), so it makes sense that an option be introduced to toggle with the following options: 1: Both horizontal and vertical tabs (similar or the same to the way it currently exists), 2. Switch off horizontal tabs, and 3. Switch off vertical tabs.
I foresee there will be a big ask from many to switch-off horizontal tabs at the very least.
Vertical Tabs
Implement like with the Windows 10 version with the Hide Navigation options available there for a clean interface with Sections and pages only available as needed.