The 4th of July fireworks are launched from all around the Seven Seas Lagoon, not just beside and behind MK. So I’d consider watching from the ferry dock at the TTC, which is pretty much a straight on view of the castle. They pipe in the music, and you’d have an amazing view of the spectacular 360° display.
That location also gives you a head start on the hordes leaving afterwards, if you’re parked at TTC or taking rideshare. To be honest, it’s not so good if taking the bus back to a Disney resort, because you’d have to ferry or monorail back to MK and be behind the crowds exiting MK. You could walk to Poly from there but would need to take the Resort monorail if traveling to GF or CR, and I’m not sure how that would be.