I sincerely hope you prove me wrong on all points!
On the boats - I'm no submariner but let's assume we opt for the Virginia class, and I've chosen Block 3 as a benchmark.
By way of comparison:
Collins class = 6 boats @ 3400 tonnes displacement, 77m long, diesel electric power, $850m per unit
Virginia class Block 3 = at least 8 boats @ 11,000 tonnes displacement, 115m long, nuclear power, $2.8bn per unit
In other words, the Virginia class Boats are over three times the submerged size of Collins.
They're about one and a half times the length, have all the additional requirements of nuclear power, and each unit is around three times the cost.
Noting your valid point about bisalloy steel - with all due respect to the engineers and yards in Adelaide, I just can't see how they'll be able to handle a project of such magnitude.
Perhaps there is a plan to build some kind of Virginia-lite, and assemble modules in different yards, splitting the work between BAE, GDEB and ASC...