Do you have an Australian Amex Platinum? The point we’re making is that you are not transferring from Australian Amex Platinum. You transfer your Australian Amex MRs to a US MR account once a year (you’ll need a US MR card to do this). You then get the benefit of 1:1 via Amex to AC, SQ, CX and others.
Then a Chase card might help you with United availability. Although I’m not an expert on that so I’m sure someone else will have a better idea on how that arrangement works.
Personally C1 and Citi just don’t seem worth the shutdown risk, especially for anyone who doesn’t have large amounts of US income, doesn’t have a US ID and has an ITIN instead of a SSN.