ok, so how is this one.
QF 433 Dep SYD 12:30, Arr MEL 14:00 (you did say leaving Thursday afternoon)
QF009 Dep MEL 15:30, Arr SIN 21:05
QF319 Dep SIN 23:25, Arr LHR 05:30 (Friday morning, arr Terminal 4)
QF3415 Dep LHR 07:20, Arr CDG 09:40 (depart Terminal 4)
In business class, that will earn 360 SCs outbound.
Now, to really maximise your return SCs and meet your timing criteria (just), how about this one:
BA315 Dep CDG 13:40, Arr LHR 14:00
BA007 Dep LHR 15:45, Arr NRT 11:05
QF180 Dep NRT 20:20, Arr MEL 07:40
QF418 Dep MEL 09:00, Arr SYD 10:20
In business class that will earn 380 SCs inbound, for a total of 740 for the trip.
But I would personally stay a little later in Paris and take this return:
BA325 Dep CDG 19:20, Arr LHR 19:45 (arrive Terminal 4)
BA031 Dep LHR 21:15, Arr HKG 15:55 (depart Terminal 1)
QF088 Dep HKG 19:15, Arr MEL 06:15
QF414 Dep MEL 08:00, Arr SYD 09:20
This will earn 360 SCs for a total of 720.
Note: use the BA flight number for CDG-LHR, otherwise you won't get lounge access at CDG. That lounge is operated by Air France and is not considered a true BA lounge and the are strict about only permitting BA business class passengers access, not QF passengers, not OneWorld or Qantas Club members.
Routing through HKG gives the use of the CX lounges there, which are wonderful, especially if you have qualified QF Platinum by then and get use The Wing First Class lounge.