How do you change Screesaver folder with command line on Mac
This works for me on a Catalina machine and is known to work on Mavericks (HT @bobsmith76 for persevering).
/usr/bin/defaults -currentHost write com.apple.ScreenSaverPhotoChooser 'SelectedFolderPath' &quot;/Users/you/sub/sub/folder&quot;

I used hot-corners to turn on the screen saver, and set different default, then when I enabled screen saver again it used the new folder.
NOTE if you have System Preferences open when making the changes then this may cause the wrong thing to happen (like the screensaver config flipping to a non-image screensaver - we both ended up on the Message screensaver this way).