Yes. 120 mm mortars, while excellent platforms, are not in the same realm as 155 mm shells. The artillery effect still needs to be replaced after we get rid of the shit M777s.
The numbers are too small. They should be quadrupled - an optimal organisation would be 3 line Regt of 3x 8-gun Bty with another Bty for training and another Bty for attrition stock. But, at this point, we can't justify that over all the other bits of kit that we need. So we'll get double the original purchase, not quadruple. It'll mean risk for the follow on Bde, but hopefully a production line in Australia will help spool up rapidly. And if you are aware of how fast the K9 can be built - let's just say delays won't be on the civilian end....
Either way, and regardless of scenario, the 30 SPG is better than what we have now. Damn, 8 SPG would be better than what we have now. The rate of fire and manoeuvrability mean that 30 SPG will be able to do more than those familiar only with towed guns think.
Also - don't forget it isn't just 30 guns out there. No matter where you are fighting. There are the long-range rockets, Air Force and (if possible) Navy assets, tanks, attack helos, mortars, IFV, cybers, electrons and a bunch of smaller rockets and missiles. Add on this a suite of sensors ranging from space to nano-UAS all joined together in a C2 network that, having come from the RA, is one of the best in the world at applying fires to the points they are needed. For us, a combat Bde with enablers and a proper Regt of SPG is a fearsome thing, probably only rivalled by 1 - 2 other units of comparable size.