I believe the issue arises when the RAAF moves away from Air Force 'bases' to various other strips in the region and having to use what's there. IIUC the ability to use 6,000' strips was a key advantage over the F104, which IIRC needed 8,000', meaning the Mirage could access many more strips in the region, even if only for diversion, transit or short-term dispersal. If you limit the Mirage to airfields where specialised pressure refuelling trucks are pre-positioned, you go back to the lack of flexibility of the F104.
I think a wide range of accessible strips
and IFR will have to wait until the Mirage's replacement, when at the very least airfield development will have improved to give more airstrip options to the RAAF.
I believe such guns were trialled but not adopted by the US. The issue with the Sheridan is its dual role of Recce AFV for Armoured/Mechanised divisions and being the primary/only Armour and Anti-tank for air-mobile divisions. This means it had to have a weapon powerful enough to knock out tanks while being light enough to be highly mobile on the ground and in the air. Without the anti-tank role, the M551 likely could have been fitted with a ~90mm gun for counter-recce and skirmishing and been quite successful in that role as expectations would have been managed.
Going with the Atar meant having to buy a whole new set of metric tools, which meant the cost advantage was reduced. You're right about the Atar 9C struggling with full intercept loads on hot days, but apparently the Avon didn't offer much improvement. The RAAF looked at the Atar 9K10 as used by the Mirage IV, but this hadn't flown by the decision date, so the 9C was selected.
Here's the book.