As noted above, it's just based on Z inventory bucket availability these days which is a revenue fare bucket and is fairly easy to search for or set alerts with ExpertFlyer and Google Flights. There's already a long existing thread here on using GUCs with post 2/1/22 changes -- Using Global Upgrades on Delta (GUCs) post 2/1/2022 Changes Note that there is also now a catch and you can only directly upgrade into D1 with a GUC from Premium Select fares if there is Premium Select cabin on the flight.

While there are currently no dates with Z bucket open on LAX-SYD flights in October, there are a bunch of dates in the opposite direction on SYD-LAX. The following is a search for 2 seats on SYD-LAX on Google Flights and the dates showing $9890 have at least 2 seats open in Z in order to come up with $9890 price for 2 seats (one-way Z fares on SYD-LAX are $4945 with 120 day advance purchase). As you can see, they are all currently Mon-Thu flights. So you are probably going to have to compromise and return on one of those days of the week. Since these fares have a 120 day advance purchase requirement, most September flights don't qualify at the point for those fares because they are too close in. The dates in September showing $11.2K are also Z fares (with 50 day advance purchase requirements), as are the Fri-Sun dates showing at $11.9K (Z fares for Fri-Sun departures are higher than Mon-Thu departures).