Thanks for all the comments. Still haven't cancelled the crappy hotel in Summer Hill, but now also have a B&B in Watson's Bay, right by the ferry, for a reasonable price. Seems Roberston's Park is a fireworks viewing venue though at ~10kms away it may feel a bit small. So we'll have to decide if to head in to the CBD and brave the crowds (by bus? ferry?) or join the local party in Watson's Bay and use binoculars
Time to figure all that out, I guess.