Can you say where you're getting the information (or on what basis you say) that 160,000 Qantas points (2 x Amplify signature cards + NAB 40k) will get you a Oneworld RTW ticket? Are you just putting together separate flights using the Qantas site and adding the points up?
Although you say you can get enough points (160,000) for $558 (or less, if you sell lounge passes), you need to be really aware of taxes and charges when booking awards, especially with Qantas FF points. For example, with Qantas - an economy award ticket from Australia - London will cost you about $400 is taxes and fees on top of your points (and even more, $600, from London to Australia). If you're using Qantas points for your itinerary, prepare to pay a LOT for taxes and charges. Another example is that fees and charges from Australia to the US can be anywhere from $300 (to Dallas Forth Worth), to $420-440 (New York/Los Angeles) - and that's one way!
Hopefully, you get the idea. 160,000 points would cost you $558 to GET. But could cost you an extra $1,000-1,500 to use if flying in economy. Considering that you can get cheap RTW tickets with Swiss Airways or Lufthansa with 3/4 stops for $2,000-2,500, I'm not sure that exposing yourself to the consequences of that sort of credit activity is worth a saving of a few hundred dollars.
Unfortunately, Qantas FF program is one of the worst going (in my opinion) due to the very high redemption rates and the taxes, fees and surcharges they add on. For me, if there is any value, it lies in their business and first class award tickets (as the fees don't change too much), although you need a lot more points.
Regarding you query about applying for 2 of the credit cards, it's worth noting that each bank usually treats each credit card they offer as a separate product. I've looked through the T&Cs for St George amplify and don't see anything that would preclude from applying for an amplify signature with each of the 3 different banks. BUT, beware that these cards have a minimum $15,000 limit each. 2 cards = minimum $30,000 credit limit. Add on the NAB card (minimum $6,000 limit) and you're credit limit is at $36,000. Just be sure you consider the consequences of that.
Best of luck! I'm happy to clarify anything regarding the above if you want.