Selfie Sticks are indeed banned in the parks.
Tripods are allowed in the parks with a couple of boundaries. They must be able to collapse to a size to fit in/on your backpack. When extended, they should not be taller than the photographer.
The most important rule of using a tripod in the park is to never make an obstruction of yourself or impede the safe flow of traffic and always be in contact with your tripod when it's propped up in the park. For example, don't set it up and then walk away to get a 'selfie' of yourself with some backdrop\.
Most of the 'good' tripod locations to shoot a fireworks show will be taken (not necessarily by tripods) hours before the show but there are indeed some good locations to consider that aren't in the scrum of main street (which requires a crew of blockers for safety.
Here are some examples from key locations:
Along the rail in front of Crystal Palace (A little old so the trees are more grown now
Right on the edge of Partners (up against the bench surrounding it)
Don't rule out the train station platform if you can camp out earlier enough to get right on the rail. This one was during the once-a-year no flagpole day when the used to tape the Christamas parade in early December.
Another options is to stand right on the curb at the end of main street under where the Christmas tree would be during the holidays. You'll have to park yourself there for a good long while in advance but the curb gives you an extra 5 or 6 inches to get you above pop-up kids that arise the moment the first shell launches