It should be possible to assign a notebook to several notebooks. And of course the note should only exist once. If a note is selected, the notebooks with the note should also be displayed. (like tags - they should also be displayed when a note is selected).
This would reduce the number of tags needed - maybe the tags won’t be needed anymore. I have to use many tags (~400) to mark notes. Some people may be satisfied with just a handful of tags, but in my case this is not possible.
Assigning a note to multiple notebooks would also solve the “problem” of nested tags.
And also the Android client would speed up, since it is not necessary to create/display the menu with all tags.
As far as I know, the data model of Joplins can fulfill such a requirement. (m:n instead of 1:n between notebook and note, like between note and tag)
Removing a note from a notebook should only undo the assignment to that notebook. The note is not deleted until the last assignment is removed.