This is what I don't get. Why do you want to stick with *A if you move to Australia, an obvious Oneworld turf? I know you are currently UA*G, you want some resemblance of continuity but the *G status will give you little benefit if you fly only about 35K miles a year with a portion of it entirely domestic Australian. If you do go with Air Canada Aeroplan, there are few things you need to keep in mind. Their miles are pretty useless at 35K level since you won't get any mileage bonus like UA gives at 35K. Plus, Aeroplan has high fees on redemptions and changes and service can be pretty bad with some long wait time. What are you going to do with your miles? And be warned, they can expire quickly.

I maintain that you should seriously consider AA as an alternative, acheiving Oneworld Sapphire will give you Qantas Club access throughout Australia and you get 100% bonus miles on both QF and AA flights. Although the requirement is also 50K/year, the trick with AA is some high end Y fares attract a 1.5 qualifcation point factor, meaning you really only to fly about 35K or so unless you fly discounted Y fares. Just make sure you have at least 4 AA coded flights per year (QF operated but with AA code works).

You may not earn a lot of miles with BMI when flying discounted Y but at least the miles can be used for something. You can get to Tahiti fairly cheaply from Australia for instance on Air New Zealand award.

Unless you can provide more details on your expected flight patterns, fare paid, and your preferences, it is hard to give you very meanigful recommendations with only pieces of information.