This is what I have done a few times and it has worked fine.
Watch Spectro Magic from near the Hall of Presidents? Liberty Tree Tavern area. Once the parade is over I always hurry (but never really needed to) over to some seating next to the 20,000 Leagues Lagoon, near Dumbo. It is comfortable and uncorwded and I have a seat to relax in. When the fireworks start, you can't miss them. However you don't get the soundtrack with this and as is obvious, the castle in the background.