Spartan vs herk is a poor example, OPV vs Corvette is more King Air vs MV-22.I would be careful about comparing Moskva to an Aegis American destroyer. I just want to elaborate so people don't think Moskva = modern aegis ship equiv.
I know what you are saying here and you are right, the Moskva has basically no networked systems, mix of ancient soviet systems, and some Russian but still dated systems, poor tactics and conops, several systems appear to be non-working or were not able to be employed because interference with other systems. Its a great example on what happens when you just dump a lot of systems onto an old large platform and don't fund any integration. Russian ships of the later soviet period always played pretty fast and loose with survivability, because if it got hit, it was likely the end of the ship, because you have expended all munitions or they had something that was undefeatable.
The OPV's would make poor platforms for basically any weapon system. We choose the least capable design for the OPV's, and then selected many equipment and modifications to further limit any combat capability. From power generation, engine, speed, Rhib placement, etc. On the spectrum of OPV's they are very much at the very basic end.
Again new new ship would be a 10+ year journey. The RAN Doesn't currently operate any corvettes, and WWII experiences aren't relevant anymore and all of those who operated them aren't just old, they have passed. So we would basically be at the very begining of selection suitable equipment for corvettes, many of the systems (SM-2/SM-6/ESSM, Aegis, MH60R, 5" etc) are in appropriate for a corvette. So unless your corvette is the size of an Anzac frigate you would be having to select, acquire, industry/logistically support, train, munitions, integrate all of those basic things, bearing in mind how smoothly the Arafura's 40mm implementation went.
For the effort to bring a small weapon system online say 57mm gun, you could be bringing a more capable weapon system online. Like 127mm advanced munitions, F-35B or LRASM. I now honestly wonder if we can bring a 40mm gun into service before 2027. Then we need to develop CONOPS, train, maintain, manufacture munitions, have a strategic store of parts, a plan for future upgrades. We could bring a very hollow capability very quickly, but how useful would that be.
We did this with the C27J.. The mini-herc concept. If the idea is the bring capability quicker, we are better off aiming for something either we already operate or almost already operate. It had commonality, and was from an existing supplier and had existing in service customers.
I would urge to consider what we can do with the existing platforms, systems and weapons of the ADF. We can increase their number, or perhaps acquire a weapon for those existing systems.
Money it costs to bring in extra Destroyers or Frigates is not that relevant in the bigger picture, and their capabilities are well known and fit in with existing planning and support. The discussion about crews is frequently bought up, but how are we going to crew a platform that is not in-service with any navy in a way we would configure it. Existing platforms, even new ones, have had people working on them for a decade before they were in-service and approaching a decade of service. Plus there are crews from overseas with two decades of service. Training and simulations are already inplace today and can be expanded, not just established.
Just looking at the RAN spec for a Arafuras there would be 9LV, but being a Corvette probably an absolute minimum of RAM, a 57mm, facilities for a Romeo, irrespective of whether it normally operated a Camcopter, and likely NSM. Sensor suite would be more ANZAC than Arafura.