Mark is absolutely right; travelling later in January and February is going to help, as it’s sort of dropping off the peak and there’s likely to be fewer higher up frequent fliers for you to contend with for that upgrade.
As a bronze frequent flier I wouldn’t say buying an economy ticket and then upgrading is a reasonable way to go if you’re hoping to score the upgrade, just because you are right down the bottom in terms of upgradability. You’d be outranked by Silver, Gold, Platinum, Platinum 1 and even the bronze fliers with an earlier join date than you. You’d also be outranked by anyone looking for an upgrade from premium to business, and anyone else who’s upgrading from the same economy class fare as you but got in marginally earlier to request it.
200k is a fair amount but as I’m sure you’re aware it wouldn’t get you both all the way in business. Might be an idea to go for a mixed class redemption, as you’d probably be able to fund most if not all of your travel using the points? Do something like economy to hong kong if you can get availability on a day flight (only 9 hours) and then try and snag business with Cathay on the long connection to London?
But yeah I wouldn’t pay for an upgradable ticket on Qantas, just because realistically I don’t think you’d have a great shot at it and then if you don’t, you’ve just overpaid by a couple of hundred to have one you could upgrade when you could have gone a discount economy fare.
Best of luck