@EricTC
I'm on a 489 visa but can answer some of your questions since they're the same for both visas.
1) I just stopped paying after I left my job in the Philippines. There is no need to inform them that you're leaving. Once you get your PR, you can go back to SSS & withdraw your funds. Sadly, we can't withdraw any of our contributions to Pag-ibig & PhilHealth.
3) There is something called a 100 point check in Australia to verify your identity for certain transactions. You can Google this & make sure you have hard & soft copies of the IDs to make things easier for you when you get here. If you aren't going to get a local driver's license right away, I suggest you get a Proof of Age card from your state just so you have an Australian issued ID with your picture & address. The date when your picture was taken will also be on the card so you can use this as one of your proofs of residency in a regional area when you finally apply for PR. The Medicare card doesn't include a picture or address. In terms of job hunting, some employers will ask for your visa details so just keep a hard copy of your visa grant handy for easy reference. Some employers will ask you to send them a copy of your visa details via VEVO, which you can quickly do online.
4) You can get any legitimate job. For 489 visa the requirement was just to have full time work for 1 year. However, for 491 the requirement is to have an income of $54k per year for 3 years, so just make sure you meet that requirement.
5) Once you meet the requirements for a 191 PR visa just lodge an application in IMMI. For 489 the pathway is the 887 PR visa. I lodged my application the day after I complied with the 489 visa requirements. You don't have wait for an invitation or a prompt to lodge the PR application.