Is there a way to quickly import all folders & files from other Notes apps (or at least complete folders in a single import)?
Depends on what you want to import. Most likely via Menu > Joplin > Import > MD directory
Import . . ./MD directory imports all the folders in the proper directory structure, but they are otherwise empty - it's just the directory. My files are all .enex, don't know if that makes a difference but . . ./MD file doesn't see them.