Completed my trip and reporting back! Actual transfer between terminals took 25 minutes, but that's because my MEL-SYD flight was on time, there was no line at immigration, and the seamless transfer bus was right there for me to board. Probably for future transfers I would be comfortable with 90 minutes (I allowed myself 2.5 hours for this trip).
The funny part was my SYD-BKK flight was delayed by an hour so despite all the stressing out about the connection, it was the other flight that was the problem.
I have to say the electronic immigration stations at Sydney are amazing! I've never seen them before.