I've used a 9.3x74R for a few buffalo with 286 gr Woodleigh PPs and it worked reasonably well.

There are buffalo on the Victoria River in the NT that can be hunted through an outfitter. Reportedly a herd of around 800 animals. No idea if they are further West.

Donkeys seem hard to kill. The one and only donkey I have shot with the 9.3 refused to fall over. Harder than any buffalo shot with it! Some beasts just seem to hang on. But I have shot a fair number with a .30-06 as well. Any of the calibres you mention would be Ok for donks.

Lots of pigs in some parts of Northern WA. Last time I was in Darwin some guys had just come back from culling over 600 of them!