There are many ways to add books, just as there are Many ways to do the Initial Management.
Make a PLAN
I would make 2 Libraries (you already have 1) They may NOT be nested.
Intake (any name will do), where you add books that will need Metadata work
Main (Calibre Library is the default, but you don't need to use this name or Library)
Add Books to
Intake (you can use
Add Books (select option) or you could drag and drop a bunch at a time.
Beat the Metadata into submission, then right click on those books: Copy to Library:
Main (delete)
repeat.
It take TIME to get it all in order