Unfortunately Andrew Ng started Deep Learning on Coursera much later :-( For sure, I would take his course.
Stanford’s lectures on YouTube are brilliant and give a background, you needed. I watched them to understand things that was poorly explained. I suppose that the Foundation ND was thought as a smooth entry point, but never to provide a deep knowledge in the areas of NLP or image recognition.
To dig deeper, in my opinion, one would better take advanced online course, practice at Kaggle or go for Master degree :-)