If you are doing regular, non-bonused spend, I think the best way to go is put everything on an AU Amex Plat and convert to US MR. It is very hard to beat the AU Amex Plat rate once you take into account currency conversion fees.
I also do a calculation of how much each point costs me. Indeed, I think that's the more important metric on some SUBs with high annual fees/spend requirements because I refuse to pay more than 0.5c/pt for points when there are so many SUBs out there.
I should have added some additional caveats:
1. As you note, a lot depends on your level of spend. For example, if you are spending $1mil pa, what you can earn on non-bonused spend is way more important than bonuses because you'll never be able to open enough cards to put it all towards bonuses.
2. It also depends a little on how hard you're willing to churn. I'm currently working on sign-up bonuses for 15 different cards (across two persons and cards in both Australia & the US).
3. I'm only talking about AUD spend. For any USD spend, obviously the maths tilts heavily in favour of the US cards.