I know you've already decided but what about a deciduous magnolia, they put on a fantastic spring show and they don't grow too big. I think it's good you didn't get the Chinese Tallowood, they can be quite weedy. Silver Birch in Sydney is right at the edge of its growing range. They need a nice cool spot and plenty of water so mabey not the best for where you're thinking? Perhaps also look at White Cedar (Melia azedarach) a deciduous Australian native that might be worth a try.

HTH

ETA – Magnolia soulangiana would be fine in Sydney (they do like a bit of water though). Heaps of amazing different flowers to choose from.