Is there a way, maybe with plugins, to combine the sidebar and notes together to be more tree-like?
Not sure about what you want to do, but I think that with some CSS could help. Please check the following links:
- First, try the Remoods Theme, a picture shows the notes tree below the folders, what allows to win some space on the screen.
- Alternative you could find the Ohmine Theme useful.
- The list of possible Joplin customizations may provide you additional ideas.
It would be great if you could let us know how you achieved what you wanted, ideally with a screenshot.