Unfortunately I don’t think it would be possible for you to get business class flights to Chile using points. You’re correct in saying you’ll need around 600-800,000 points to fly to Chile return. It would be incredibly difficult to get the 600,000 points you need in a short time period.
However, even if you had enough points, Qantas makes very few business seats available using points to Santiago. Over a two week period, they might make 2-3 seats available. And by that I mean 2-3 seats on the 14 flights in a two week period, not per flight. There is very little competition between Australia and South America and as such, the airlines tend to be able to sell business seats for cash and don’t need to make any available to use with points. Trying to get an upgrade using points is also highly unlikely for these reasons.
Sorry it won’t be possible. It was a shock to me when I first learnt how many points it costs on Qantas to get business class tickets and the fact they’re so difficult to find!