"I think a 3rd option of either the new SSN(X) or some form of bespoke "regional sub" for countries like australia and possibly SK, Japan or india"
Problem is in the execution - the UK, USA & France have been building SSN's etc for nearly 70 years and they ALL have problems with every new design
trying to build up expertise from scratch is a very long and incredibly expensive road - and there's absolutely no point in building something that's as good as the USN Nautilus in 2030 - you have to be close to world standard from the start. Buy or rent the first few boats