FBW for subs is a good option if your concern is the Malacca straits and the access to the Indian ocean, the gulf and the important oil trade that happens there. The US struggles at this point to project power, its not ideal for them. India, Malaysia and Singapore aren't exactly US allies in the true sense, they are non-aligned. There is a reason why the US based subs out of Perth in WW2.
The east coast of Australia is good for projecting power to the wider pacific and fending off fleets from the East coast trade routes and the outer SCS islands. Which is why we built the original sub base there. While the east coast looks pretty blank, look at actual shipping routes and bathometry and key spots appear.
Download scientific diagram | Bathymetric map showing key sites for Australia and South East Asia showing sites mentioned in the text. Underlying topographic and bathymetric data from gebco 2014 (www.gebco.net) and Esri, DeLorme, HERE, TomTom, Intermap, increment P Corp., USGS, FAO, NPS, NRCAN...
www.researchgate.net
Just how exposed is Australia to the dangerous dance in the South China Sea?
www.smh.com.au
From the east coast you get a lot of deep water access into the SCS rather than from FBW where its all shallow stuff (or time consuming) depending on which way you want to go.
Ultimately two bases gives a lot more option and makes it much harder to track and counter Australia's submarines. Australia also tends to operate with the US and the US has most of their assets and facilities in the Pacific. Also there is ~20 million people living on the East coast verse almost 2m on the west coast.
Momote/Manus is a nice spot, far enough away that its not in immediate threat sitting just on the edge outside of China A2/AD. Plenty of deep water, plenty of nearby trade routes going to China/Korea/Japan.
