Although this post is mainly about figuring out how to get CBR > SYD leg added on to a UA ticket, I thought I'd share some knowledge on how to maximize points for business class travel with them, particularly given how difficult it has become to find saver Polaris availability over the past year.
Just thought I'd chime in as someone who pre-COVID was flying United a lot and often times in Polaris business class. Your best bang for your buck in terms of using those points IMHO would be doing a points upgrade from an economy fare to Polaris business class. In particular, my last trip with United which was late January 2020 was booked as a $1200 AUD LHR > SYD return ticket. I requested a points upgrade to Polaris business class for 35,000 miles + $650 in taxes and fees and the upgrade cleared at check-in for both legs (LHR > LAX and LAX > SYD) which is not bad for some 25 hours in the air + 5 hours at the Polaris lounge at LAX.
One nice thing about United's upgrade system is that unlike Qantas they can upgrade you to Polaris at the time of booking, supposing there is inventory available. By pulling up the advanced search option United provides, you can request it to lists results by MilagePlus Upgrade Awards:
Then you will be presented with your cash fare flights as usual but this time with details on upgrade availability:
In this case as you can see an upgrade to Premium Economy is available by simply purchasing the cheap economy fare then spending 30,000 points to sponsor the upgrade. If I was you though, I'd avoid Premium Plus and instead focus on Polaris, as on UA's 777 and 787 those over-wing exit seats in Economy+ are best!
So that's the first thought. Now if you really want to get business class seats, don't care about flying United but just want to get those seats at the lowest price, my recommendation would be using the 30 day calendar option on United to see where the cheapest award tickets can be found. This can be done by doing a normal award search (but choosing flexible dates) and performing the search. From there you will see a 7-day award calendar just below the departure dates:
Click the View 30 day calendar link and you will be presented with a calendar view showing you the cheapest awards available for each date:
As you can see for the dates, there are two days in July where United has cheap business class awards: Thursday July 7 and Thursday July 21 for SYD > SFO. Viewing the 7th confirms this with SYD > LAX being Polaris J and LAX > SFO being in Economy, which isn't too bad considering you would still have Polaris lounge access on arrival at LAX:
Hope that provides some guidance and apologies if this post was a bit rambling.
-RooFlyer88