It is currently not possible to list all top-level notebooks or to list all notebooks that are nested within that folder using the joplin terminal command.
I believe this would give joplin amazing flexibility. (Basically my goal is to make all notes searchable with fzf, so I do not have to enter the app if I want to read or edit a note.)
I wonder if we could create a new command that would basically plug into the existing API interface (like the plugin data API is doing). That way you'd have full access to everything within one command.
But otherwise for now you could use the command joplin server start to run Joplin as a server and use the API, but it's not as convenient as a command.