NotificationEmailEvents class not found
The docs at the time of the question reference an old classname in the C3 example, the two Notification Email related classes in your code are now in a module named sprout-base-email:

NEW
use barrelstrength\sproutbaseemail\services\NotificationEmailEvents;
use barrelstrength\sproutbaseemail\base\NotificationEvent;

OLD
use barrelstrength\sproutbase\app\email\services\NotificationEmailEvents;
use barrelstrength\sproutbase\app\email\events\NotificationEmailEvent;