At 3,600 tonnes or ~4,000 tons, the ANZAC-class FFH is about 1,600 tons greater than the suggested corvette. That would strongly suggest a fairly significant capability gap between the two. I agree that further improvements to the ANZAC-class would be difficult to accomplish, partially due to the size and displacement, but also due to the layout. If the MEKO 200 design used had the Mk 41 VLS located immediately aft of the 127 mm gun firing from the same deck level instead of immediately aft of the funnels and two decks higher up, then there would likely have been less stability and topweight issues.I would rather we go the other way. An all cruiser surface combatant fleet. Lengthen the Hunters ~10m pushing them up to 10,000t. Providing space for more VLS, more sensors, more command and embarked, more flexibility, more power, more range.
The ANZACs are at the very limit of what is capable in a smaller hull. We already have those. A less capable Corrvette some 500-1000t lighter doesn't seem to solve any of our problems. We also have the OPV comming up, at ~1700t. So its a pretty small niche to hit with a corvette. As we consolidate roles with the OPV coming on line it is possible we could make a batch of some improved OPV's. Suitable for heavy piracy, amphibious insertion etc. But are we better off rolling that into more Hunters.
If the OPV's are going to end up getting tasked with more of the pointy ended constabulary missions than just EEZ and anti-SIEV patrolling then armaments and defences more substantial than a 40 mm gun and some M2's are going to be required. However, that would still fall well short of an OPV or corvette being adequate to screen/escort high value and/or civilian shipping that could be subject to targeting by hostile aerial, surface, and/or subsurface threats, which is what the current major warship fleet is intended to do.