Both DFW and JFK are easier to find seats to SYD. Sometimes you can find JFK-LAX-SYD but you would not find LAX-SYD (or BNE or MEL) because of QF's married segment which is to ensure folks from East Coast also have a shot on the award seats.

LAX is the hardest on both directions.

It may boil down to just where you can find the J or F seats to go instead of basing on where you prefer your stopover.

Stopover is always the gateway - being the last city leaving NA or the first city entering NA.