Is there an efficient way to give a large class of students feedback about their assignments?
If the class has 700 students there should be a lot of students who are making the same mistakes. Therefore, I do not see much benefit in personalized feedback due to the impracticality of the context. 

I would consider it wiser to determine what are the common mistakes that the majority of the students made and provide the same feedback to all students on how to address these common mistakes. This will help the majority of the students without having to commit to the time-intensive effort of personalization.

If there is someone who wants additional explanation they can speak with the TAs.