Re a simple tool to organize photo based on date

Terminal: diff

You can compare two folders in Terminal using the following command:

diff -rq /path/to/folder1 /path/to/folder2

To ignore folder hierarchy you can copy all the files out of a folder into a third folder, then compare these:

find /path/to/folder2 -mindepth 2 -type f -print -exec cp {} /path/to/folder3 \;

You could upgrade your iCloud storage and utilize the iCloud Photo Library, this can be used/setup by checking the respective box in Preferences.

The other option would be to have your Photos library on an external hard drive. Then when launching Photos, hold the option key, choose other library on the bottom left, and you can point Photos to look at the external drive's Photos library. This will only display your library when your external drive is connected however.