If you consider that most cards offer around 1 FF point per dollar spent (you can get 1.5 on some cards), and if we take Qantas FF program as an example you will need 64,000 points each way (128,000 return), you will need to spend way more than your 20K income in order to earn a reward flight.
Credit Cards are not a suitable means of earning free flights via airline FF programs unless you can channel large amounts of spending through them, and that is usually done by people spending other people's money rather than their own.
Also consider the fact that you will have no status in the FF program, so availability to FF award seats will be limited and require significant lead time to secure the seats.
You may be better off with a card that allows you to use the points for a voucher that can be spent at a travel provider. But even still, with a low volume of spend the benefits will be marginal at best.
For example, Amex allows Membership Rewards points to be used for travel services from Travelscene at a rate of $100 for every 12,000 points. So for a rewards Maximiser card that is earning 1.5 points/A$ spent, you need to spend $8,000 on the card to get a $100 saving on services purchased from Travelscene. And the card will cost you $195/year. So you need to spend $16,000/year on the card to break even on the rewards benefits.