Hi all, thought I would provide an update for anyone else who might be in a similar situation, after having made it through the border and into the US.
Firstly, at the airport in Australia I was given my boarding pass no issues. The rep at the counter did spend some time speaking to his supervisor and to someone over the phone (not sure who this was as he was standing away from me, maybe CBP?) before issuing the boarding pass to me.
After arriving at LAX, I was asked why I travelled with an expired green card and no ESTA. Once I explained my situation I was taken for secondary screening, at this point the officer told me I shouldn't have travelled with an expired GC (even though the 2021 CBP document says that I can with a 10yr GC) and that he believes I have no intention to permanently settle in the US. He then provided me the documentation to fill out the I-407 form and told me to abandon my LPR over the coming weeks, which I intend on doing as I have no plans to ever settle in the US. After that he allowed me into the US as an LPR (no I-193 required).
Finally, with my wife who did receive her ESTA even though she never filed an I-407 so is technically still an LPR, they let her through with her ESTA no questions asked. They didn't even ask to see her GC.
Hope this helps someone else in the future.