7:20 is a perfect time! Ours were at 7:30 last year and we had finished our deserts and it was just about time for the fireworks to start. The balcony was closed because it had rained right before our reservation, so when Illuminations started, they took us down to the empty balcony and we had front row seats. It was great!
If you are worried about the time...just tell your waiter that you want to have a slow, relaxing meal and watch the fireworks afterwords. They will greatly help you with that!