All of the above is good.
If your books have ISBN numbers then that alone is (IF CORRECT) all you (USUALLY) need as the other metadata can be downloaded automatically from the ISBN.
There is a Calibre plugin that automatically searches text ( but not image) based PDFs (?and other text formats) for an ISBN. It does a good job however it can be misled by text referring to another book's isbn.
You may need to consider how critical correct metadata is for the library. For example I wouldn't have faith in my library a metadata in a life or death situation.
If after using all the automated processes you may find that renaming individual files to reflect their author - title by keyboard is quicker before adding them to Calibre. Bulk Rename Utility and others will let you rename using a list of "old file name|new file name" pairs.
I'd interested to hear how you go and what work flow you find for your task.
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