This is very hard to answer. As a plat you are near the top of the pile. As to routes and planes, probably the QF1/2 is historically almost always rammed, but so are most planes at the moment, and esp over Xmas/new year. It is a space available upgrade. And QF will happily let it go out with free seats even when people are willing to upgrade.

if you are happy with flying Y and see the upgrade to J as a great Bonus.

I would though suggest you head over to the BA forum. There are lots of discussions there about flying other less known airlines, and ex EU routes esp from Scandinavia that can be much cheaper and may put paid J closer to your budget. Look for ex EU discussions or SYD discussions which invariably go off topic quickly on the cheapest ways to AU which is rarely QF/BA ex LHR.

KF