Photos Library Space Usage
A late thought… which might depend on how much 'organisation' you'd be willing to lose.

If you right-click the Photos Library &amp; select Show Package Contents, then inside there, the Masters folder contains just the photos. All of them, but nothing else.
Everything else inside the library package is 'organisation'.

You could test creating an entire new Library by  


copying the Masters out for safety
Launch Photos whilst holding  Opt ⌥  
choose a new Library  
Importing the Masters folder you just moved out.