I think this AUKUS nuclear submarine deal is just one part of a bigger picture. We can discuss at length about what good those 8 new nuclear submarines would do for Australia if they do come by 2040. The bigger story should be this AUKUS alliance itself.
The US and the Western media is touting the formation of yet another anti-China alliance in Asia centered on AUKUS. It stinks of Anglo-Saxon superiority and colonialism. Nevertheless, I think in the future we could see the Quad eventually merging with the AUKUS. Though doing that right now is only adding just one UK into the existing alliance. The US will definitely try to add more members into this alliance. Then the Quad will be renamed into something else, with AUKUS acting as a HQ of sorts in Australia, like Brussels for NATO.
From a US strategic POV, having Australia become a big naval base for the US Navy makes sense. Australia is much further away from China than Japan. Plus there is the added advantage of language and cultural familiarity with the Australian people. Many things could be based in Australia. From naval fleets, to long range bombers, missiles, ground forces, and God-forbid, nuclear weapons via a 'nuclear sharing program'.
With regards to the Malacca Straits. Geographically, Australia has direct access to the Indian Ocean, so in case of anything happening in the MS, their seaways from the Indian Ocean is gonna be relatively well insulated. The same cannot be said for Japan, SK, and ASEAN. Forget about India, if the MS is gonna be blockaded, it'll be led and done mostly by the US. If the US is mad enough to do that, then they, and their friends are already prepared for war. That is why there is a strong determination by AUKUS to decouple from China. Its a kind of preparation of sorts. Other countries can't do that, but that is not the US's concern.
For ASEAN, AUKUS is just not good news. It just does not have ASEAN's interest at its core. Nothing that AUKUS does is gonna benefit ASEAN. They aim for more militarization of the SCS, and the dismantling of China. Both are just bad for ASEAN's development. Unfortunately ASEAN is not in a position to pushback against this US war plan. They are not united enough, and not powerful enough individually to do so. China must take the lead here to do something with AUKUS.
Frist, China would need to speed up projects that help to alleviate its over-reliance on the MS. The priorities are the BRI projects in Central Asia, China-Russia hydrocarbon pipelines, Xinjiang oil-field developments, and green energy developments. Second, China needs to get closer to Indonesia to counter Australia's naval ambitions. Indonesia has its own problems with Australia, and is the only ASEAN nation who has direct access to the Indian Ocean. Perhaps get Indonesia on-board into some kind of naval alliance. Not directed against any country, but for something far more important: to keep the sea lanes open for trade. At the very least, get Indonesia to allow for the resupply of PLAN ships at Indonesian ports.
On the military side of things. The AUKUS nuclear submarine deal should give China the kick it needs to finally speed up their ASW and nuclear submarine projects. The PLAN has formidable ASW capability right now, but it still needs further improvements. Newer classes of SSNs, and SSBNs are long overdue, and are much needed right now for actual serious deterrence against the US.
The AUKUS may be overhyped, but the US is currently trying to follow its 2016 RAND Report for a war plan with China. It is a cynical war plan. A US military victory is not the priority. Its primary goal is to create a major war to destroy China's economy, and bring about another Soviet-style collapse. It is an apocalyptic plan, but its no longer unthinkable for the US these days.