My Photos library is already getting somewhat hard to manage with only 50000 photos and videos, 250GB.
I am continually exporting partial libraries to keep the library small and are only keeping my favorites and the new photos and videos that still need editing in the System Photos Library. All other items are archived in small libraries, one library per year, plus one library for each major photo shooting event, like a photo safari in some national park.
I am using Power Photos to export and merge selected folders and albums to a different library.
When I want to split a large library, I create two smart albums, one smart album to show the items that should remain in the library, one smart album to show the photos that will get moved to library two.
Then I duplicate the library and delete in the original library the items in the smart album for the photos to be moved to library two. In library two I delete the items in the smart album with the "keepers" for library one.
The smart albums will me help to split the library correctly, without deleting photos from both libraries.