This works for me on a Catalina machine

    /usr/bin/defaults -currentHost write com.apple.ScreenSaverPhotoChooser 'SelectedFolderPath' "/Users/you/sub/sub/folder"

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.

If you need to adjust values in the dictionary element do like this (setting name=XX in CustomFolderDict)

    /usr/bin/defaults -currentHost write com.apple.ScreenSaverPhotoChooser CustomFolderDict -dict-add name XX