Hi Emily,
If you are wanting to enjoy a table-service meal, I would recommend making an advanced dining reservation for at least 2 and a half hours prior to the scheduled time for Fantasmic on the day of your visit. I always allow an hour for each table-service meal and would suggest that you are waiting in line for the performance at least 60 minutes prior to the start of the show.
If Fantasmic is scheduled to be taking place twice on the date of your visit, the second showing will almost definitely be less crowded than the first.
Another option would be to grab a meal from a Quick Service location and eat it whilst you are queueing up or once you are seated. My family have done this many times in the past and if nothing else, it helps to pass the time whilst you are waiting for the show to begin.
I would also keep checking the online dining reservation system
HERE for availability. Cancellations do happen and you never know your luck.
Wishing you a magical trip!