My money is first RAN subs will be SSN(R/X) they will have a tripartite reactor
Congress is currently debating whether to make the design of future reactors (SSNX onwards) use LEU rather than HEU. That’s a decision which would lead to a 15 year design orogramme.
One if the main advantages of using a PWR3 reactor is also exportability. exporting a U.S. reactor will require Congress to change the law, something it may not be willing to do. the UK won’t have that problem.
LEU reactors would require mid-life refuelling - a reason which led Australia to discard the option of French nuclear Barracuda subs as it would take away strategic independence for the life if the boat and require several years in a French/US yard to perform per boat.
The above to just to make the point there are no easy low risk decisions involved.
SSNX will also be Columbia size and cost around $6B apiece.
https://tinyurl.com/3epaamnw
https://crsreports.congress.gov/product/pdf/IF/IF11826