Paphitis,
Few points came to my mind today regarding your situation with the E-3 visa, which might have negative issues which you might not face if you had a Green Card.
If you start working in the US, you will be in the max income tax bracket with your expected income. You will be in the 37% bracket more than likely. Then you will have state income tax, usually around 5-6%. Then the usual state sales tax which are around 10%. Some states do not have state income tax or sales tax. California has all of them.
Then you will have about 6% Social Security tax for your state pension, which is what I want to talk about. In order for you to qualify to get any SS benefit when you retire, you will need to have at least 40 credits, which means, you can only get one credit per quarter, which makes it 4 credits a year, which means you must work for 10 years minimum to qualify. As long as you are working in the US, no problem, however, with your E-3 visa, should the company decides to lay you off after 8 years and you have not found another job under the E-3 program withing 10 days starting the new job from the last one (according to the link you have provided earlier), you will basically have your E-3 visa cancelled I would presume. My point is, for whatever reason you do not complete the 40 credits, you will not be able to receive any SS benefits when you retire, say in Australia and all the money you and your partner have put into it, will be lost if you are told to leave the the USA before you get your 40 SS credits each.
The other issue is the company pension, which these days you and the company pay towards it. Find out how many years it would take to be vested in their company pension as each airline is different depending what the collective bargaining the Unions have with the company. It could be 5 years or more. Often we overlook these little things when retirement is still far away for some, but you need to be on top of these important issues. So, aside from looking for a good immigration attorney for a Green Card issues, I would also recommend you looking for a Tax expert for the US to advice you and your partner regarding these issues. All these will add up to thousands of dollars, so make sure you do not lose them by oversight. If you get a Green Card eventually, any kind of work you will do if not flying under E-3 program, you can build your minimum 40 credits as there is no time limit until you retire, so that you will not lose your SS benefits, even if you retire in Australia eventually.
Here are some info.
One Social Security credit will require more earnings
In order to qualify for Social Security benefits, you'll need to earn 40 Social Security "quarters of coverage," which are also known as Social Security credits.
For 2019, one credit translates to $1,360 in earnings, an increase of $40 from 2018. This may not sound like a lot of money (and it isn't, for most workers), but it's important to note that you can only earn four credits per year.
The maximum Social Security benefit will be $73 higher
Finally, because Social Security benefits are limited by the maximum taxable earnings from each year (which rises over time), the maximum possible Social Security benefit is increasing for 2019, as well.
For 2019, the most someone claiming Social Security at full retirement age can get is $2,861 per month. Of course, this could become even higher if workers entitled to the maximum decide to wait longer to claim. If they claim in 2019, a beneficiary's full retirement age is still 66 years old (born in 1953), so if they were to wait until they turn 70 in 2023, their benefit would start at $3,776.52 plus any additional COLAs that are given between now and then.
Tax brackets 2018-2019
Taxable income Tax on this income
0 - $18,200 Nil
$18,201 - $37,000 19c for each $1 over $18,200
$37,001 - $90,000 $3,572 plus 32.5c for each $1 over $37,000
$90,001 - $180,000 $20,797 plus 37c for each $1 over $90,000