I would suggest that crediting to AA might be an even better option. I wouldn't recommend crediting to QFF these days due to the ridiculously poor award values

If you credit the trip to UK to AA and credit to AA and undertake the Platinum challenge, you will attain OW Sapphire on the outbound journey on its own. Mileage earning will then have 100% bonus

SYD-LHR-SYD in business class credited to AA and taking the challenge will get you

21146 base miles
21146 bonus miles
5288 class of service miles
Total 47,580 miles
plus 3180 miles for sidetrip to vienna

for 50,760 miles

If you then book SYD-ORD through AA using the AA flight numbers you will then earn ( in economy )

SYD-LAX-ORD is 9232 miles so will earn a total of 36,928 miles from that trip

Adding that to the previous trip will be on approx 87,688 miles

60,000 is enough miles for a one way business class award flight to London
80,000 is enough miles for a one way 1st class award flight to London
90,000 is enough for an economy return to UK

compare that with the 128,00 and 192,000 miles needed for a Qantas award one way flight to London ( plus about $700 in additional surcharges to pay ) in business or 1st class

Also as a Platinum AA member you may ( subject to availability ) be able to get the LAX-ORD-LAX flights upgraded using the upgrade credits earned from the trips