I’m no expert , but will try and help.
Having a look around the dates you wanted there is literally no business class available, so i came up with a few alternatives that may be of interest.
Using the dates of May 11 - May 22 (2PAX)
Prem Econ Sydney to Japan(NRT) + First Class/Suites Japan to Sydney
P/ECO - SYD>NRT: 114,750KF + $564.84 (777-300ER BOTH LEGS)
F/C NRT>SYD:161,500KF + $553.62 (777-300ER(NRT>SIN) A380(SIN>SYD)
TOTAL 276,250KF(372,938 Velocity) + $1,119.46 taxes
you will need to book this at 2 separate legs as you cannot book a return multi-class flight (although i have read some people have managed to do it via the call centre)
another option is First both ways
same dates as above
777-300er sydney to Japan & Japan to Singapore
A380 Suites Singapore to Sydney
TOTAL 323,000KF(436,050 Velocity) + $1232.00 taxes
if was me i would go option 1. Prem Economy is still fantastic and you get the added bonus experiencing First Class Suites on the way back! + you will still have around 225,000 Velocity Points spare to use for your next trip.
P.S I wouldn’t leave it too much longer to book a) the flights will go pretty quickly and b) singapore are removing the 15% point discount when booking online i think on the 23rd of March
Hope this helps! let me know how you go