One way to donate $7000 to the Australian government is to pay for an application and then not submit any supporting documentation.
The police checks and medicals do not need to be submitted straight away, and in fact immigration states on the website to not upload these things until requested as they have a validity period of only 12 months and visa processing can often, especially for 820s, take longer than that.
But everything else? You absolutely should get that uploaded straight away. If they go and look at your case and see you have no supporting evidence, you'd be lucky not to get a refusal straight away. It's not like you're missing one item and they did you the courtesy to ask for it (which, by the way, they do not have to do).
And yes, the BVA can take a few days so you want to give yourself a couple of weeks minimum before the current visa expires. We paid by credit card and payment went through immediately and so the BVA was granted immediately. If you pay by BPAY and it takes 4 or 5 days to process, or there is an issue and it doesn't go through, you don't want the other visa to run out while you sort it out. But generally, the BVA can be received within hours.