im not so sure that I agree with not doing extended responses for each topic. If you have time and are really struggling with extened responses, reading band 6 responses is great, but other than that I cant see a better way to improve other than sitting down and actually writing some responses yourself.
if you really want to improve then you could write an essay for each topic, but you might want to just focus on big essay topics. like monetary and fiscal policy, globalisation, etc. write a few of these essays refine them to 18+ standard and then make them flexible. for example, micro and macro economic essays can be prepared so that you only really have to change your essay a little bit to fit the question, depending on what they ask you to focus on/stimulus, etc
just keep at it and perhaps try to plan out your essay for 5-10 minutes before you attempt them at home. hopefully in time you will be able to plan them in just 2-3 minutes before you start writing
I think that writing practice extended responses is different to writing prepared extended responses. Practice extended responses are great. but they are really only useful when A) you don't know the question before you attempt to answer it B) you do it under exam conditions, or within similar constraints.
Otherwise this idea of "preparing" a perfect answer to a question that you will never get is a waste of time, because you could prepare 100 responses, but you can pretty much garuntee that no question you get in the HSC will match a prepared ext response.
Particularly at the time of the half yearly and approaching the trial, the best thing to spend your time on is to be studying the actual content of the course. Most students are not 100% with some of the concepts and there still needs to be attention to detail. Exam strategy is still very important at this time of the year, in fact it is probably more important now than at any time of the year, but thats precisely why prepared essays is not the answer. Students need to learn how to write an extended response under the pressure of the HSC without having to rely on a pre written plan.
This is why reading band 6 responses is important, because there are common elements between all top performers. Naturally when you read enough of them you start to emulate this yourself. It is also good to read some lower band answers so you can compare.
So bottom line is practicing extended responses is a good idea, particularly when you know the course back to front (usually occurs after the trial after you have studied everything), but preparing answers is not.