One option that ticks a few box’s is early lease of the decommissioning Los Angela’s class, that have a few more years left in them, with large amount of US crew who want a posting to Perth, so we can start the learning curve, and free up Collins class for refurb for extended life.
I presume US does not have spare Virginia class hulls to sell, nor a lot of spare capacity to increase production rate for an early off the shelf purchase whilst Aust gets it’s act together.
It would also keep our crews using US combat system (Raytheon CCS Mk2 (AN/BYG-1)) that RAN are so keen on.
So lease of old boats is a way to get foot in door. Somewhat like what India has done with Russian nukes.
Have to overcome some RAN resistance to old ships given experience with Kanimbla and Manoora, but RAN will be keen to do anything to facilitate getting their foot in the door of nuclear subs so it becomes real rather than a future proposal that someone can cancel.