I have a saver award booked going out to SYD (December 2023) and there is an opportunity to upfare from coach to Premium Plus for a differential that is smaller than the fare differential you have stated - note this could change at any time. I have not pulled the trigger yet as have not been able to get anything reasonable for the return and the upfare is non-refundable whereas the miles can be returned. My upgrade last December (2022) to SYD was very touch and go from paid coach to Polaris (#9 with 10 seats available a couple of days before the flight) and I ended upfaring to Premium Plus the day before and then upgraded with PlusPoints to Polaris. I think only 6 or 7 people got upgraded for the flight so being in Premium Plus definitely helped me get the Polaris seat. I am a 1K member and it was tough for me to upgrade to Polaris and the only way it happened was to be in Premium Plus.

I am sharing the above so you can manage your expectations. I will say that Premium Plus is better than coach for that long flight (but Polaris is obviously even better for the lie flat bed).

Good luck - Christas and New Year in Sydney is wonderful!