You can break the contrack but IMO its unlikely to be cheaper (as you seem to want to have a plan and phone going forward still?)
You won't get an answer here. Contact Optus Live Chat 24/7. Get the figure that applies to your contract. Some have no exist penalty some do.
Regardless you'll need to pay the handset component, and as Optus are subsidising your handset cost each month, when you cancel they stop subsidising. Optus won't buy any handset you have, although some plans have a $99 fee and you can change your (perfect condition) handset. You'll be placed on a new contract and if you choose a cheaper phone then possibly your monthly fee will drop to $80? for the next 24 months.
IMO you're probably best off cancelling the contract. Selling the phone. Buy a cheap ($200) phone and get a prepaid plan that you can up or down each month until your finances have stablised. But crunch the numbers.