Glad I could help.
I find working backwards often easier. Using

• Wikipedia for the airport. Tend list airlines & routes. Very simple format
• Wikipedia for the airline destinations. Very simple format. Kept up to date for some airlines. Poor for others
• Airport web site arrivals info. Some have code shares in a simple to read format
• Airline web site schedules/timetable. Many airline web sites as so full of glossy - advertising *****, that finding the routes they fly and when hard to find.
• Published new/restarted routes in the future can be hard to find. Can have good A D L *ONE* availability.

Keep a watch on the QF flight, They currently have a tendency for change.

Also worth keeping a lookout for a better LHR-AU D flight. Something may come up. Including on code shares
Example QF82 SIN-SYD is also AY5011 & UL3372 so could be used on *ONE* if has D and if an agent can be convinced to use.
Example QF2 SIN-SYD is also AY5003 & UL3392. Possibly LHR-SIN-SYD
From https://www.sydney-airport.net/syd-arrivals-terminal-1