Surely the answer is to raise, train and sustain more crews and accept we need a bigger fleetTBH if the concern is about the viability of the ANZAC-class frigates into the future, I think the RAN would be better off pushing to accelerate the Hunter-class build rather than come up with a new/another vessel design to be built and commissioned into the RAN. From my POV, the concerns I would have about this makes no distinction between a 2nd class which is also built in Australia, or an overseas build.
Yes, having a 2nd class built (if it could be done in time, which might be possible by then again might not) could let the current frigates be decommissioned early/earlier, but there are a number of potential issues with doing so.
As currently structured, the RAN is setup to operate and support about a dozen major surface combatants, more or less. If 2nd class of vessel also gets built for the RAN, in addition to the Hunter-class being built and the existing Hobart-class, the RAN is still only going to be large enough to operate and support ~12 combatants. If three or four hypothetical vessels were built in a programme alongside the Hunter-class build programme, then once ~nine vessels between the Hunter-class and 2nd class were built, then the new construction for the RAN would need to stop, otherwise the RAN fleet would get overbuilt and there would be more ships than crews available, likely by three or four frigates.
This would leave the CoA with some unpalatable choices to make. Either cut back the number of Hunter-class frigates to be built down to five or six, thus shortening the build with construction for SEA 5000 likely ending by 2038 if not earlier, or keep the number to be built at nine, and then retire the additional class of vessels quite early in their careers, after spending all the coin to have them built in the first place.
If the number of Hunter-class frigates is cut down to ~six because another class also gets built, that would impact national continuous shipbuilding regardless of where a 2nd class build was done. By current numbers, the current plan was to build through to 2044 which would provide steady work to sustain shipbuilding whilst also have enough RAN personnel to crew vessels being built or otherwise in service. If the RAN were to have the ANZAC-class frigates rapidly replaced, there would be no follow-on shipbuilding for several years because the current RAN needs and capability to operate ships had been met. It might be possible to launch a replacement programme for the Hobart-class early (as in, get it started now to be ready in 10-15 years for production to start) but that would likely only be a three ship build programme which would finish in the mid to late 2040's. This in turn would likely cause a ~decade gap in shipbuilding because the first of the Hunter-class frigates likely would not be due for replacement until in the late 2050's or later.
It is unfortunate that the RAN finds itself in such a situation where there are no real good solutions. Solutions which solve certain problems end up creating other problems. To make matters worse, any potential solution would need to be one which the RAN could apply and sustain for years, likely through several changes in gov't. Given how long Australia has gone so far into the continuous national shipbuilding strategy and following a change in gov't it appears that a current programme which is a key part of the strategy is being looked at by gov't to be possibly significantly changed/cut short, it leaves me less inclined to believe that support for a nationally important strategy is strong enough to make it work.
Obviously this means more dollars, but we have 10-15 years to solve for this and a looming recession to recruit from. The RAN needs to be much larger in terms of manpower.
As others have said, an expanded East Coast presence would help. FBE looks like it’s close to capacity, so the SSN facility in Newcastle or Wollongong would help. But we also need something which allows crews to live and work in Brisbane and Melbourne while ashore. I have no idea of the logistical and operational complexity of doing this, but would it be possible to have, say, a Hunter and a Tier 2 (or at least a roster of crews for them) home ported in Brisbane and Melbourne? Does it all have to be Sydney and Perth?