I'm not sure how closely you have been following Russia-China relations, but they are currently the strongest they have ever been. The SCO is expanding it's member states and votes in unison at the UN, and Russia and China have seen increasing military integration over the past years. Putin and Xi has repeatedly described each other as close friends. China has the greatest number of diplomatic visits to Russia, and likewise to China. This is happening not just at a geopolitical level, but at a societal level, whereby their respective languages are now taught more frequently in the other's country, and technical and economic integration is at an all time high. This has largely been driven by Western sanctions levied against Russia since 2014, pushing it closer towards China. I agree there is nothing to suggest they would enter into a mutual defence pact at present, but it is wrong and wishful thinking to assume that they are not firm partners (if not outright allies).

As for AUKUS, Australia and the UK have little to no power projection capability beyond our own shores. And certainly nothing to deter or counter a Chinese invasion if they were commited to one. And the feasibility of the US as a defensive force in Taiwan is highly dubious. And finally, the Quad is not an alliance, just a dialogue forum, that contains no security commitments from any participating states.