At least do another year in the degree. At worst it will give you more RPL when you join the cadetship (which Swinburne will give you). At best you will see what happens with the QF/JQ financial difficulties and you can stay there.
In terms of "should I do a cadetship?" google, search here etc. It's a personal choice. I personally had a bit of both and I would say, GA is great, for an 18 year old it is as fun and adventurous as taking a year off and backpacking Asia. As someone who was young getting into aviation. Enjoy the journey and the lifestyle, you don't have to worry about wives, kids, mortgages etc. for many years, have fun and make the most of the freedom. Trust me, ~20 on a 210 is very young in this country. Spend a few years kicking around GA and you will still get into airlines younger than many 'cadets' who are career changing.
*all advice is based on my circumstances and the current employment conditions. Stuff changes all the time and we are all an inch away from centrelink. Happy Flighting!