You really don't have to "explain" any of that to me. It may just relate to my day job......
The debacle of the post T/V boat skills shortage hit the design element first, then the production. Which is why Barrow ended up building some surface ships and a couple of commercial tankers in the late nineties / early noughties just to pay the bills. Now they have a continuous stream of production work, that side will take care of itself to a degree. What is less obvious is the design cycle, particularly for the initial stages, which for AUKUS has been going on for quite a while now. One of the reasons to decrease the build time will be to potentially decrease the interval between design times as well.
I did say it won't be easy - but that's also why the overall arguments on numbers vs capacity need looking at. One of the other consequences of long build durations is that much of the kit is legacy by the time the boat gets into service. That makes the support function even harder.
However - given the squirming over the actual budget trajectory I doubt Starmer has any answers. This is a kick the can down the road budget.