The spread itself isn't that bad. I've noticed it can be as low as 0.2-0.5%. Conversion fees can also be reduced if using IBKR. Wise at 0.4% isn't too bad either.
Overall I think you can reduce the average loss from the spread and conversion to about 0.7-0.8%.
However the biggest loss that is difficult to manage is the volatility of the USD/AUD rates.
For example, I used my US amex last month to make some big purchases when the AUD was at 67c USD, then in just 2 wks it crashed down to 65c and I needed to convert to USD at 65c rate to repay my AMEX credit card. From my calculations I made about a 4% loss off a $6K purchase and now the AUD rate is back to 68c USD.
Its also a massive headache on deciding when to convert, will it drop further or will it go back up, the cc bill is due next week etc. Playing with FX rates to me is feeling like a mugs game right now.
Can convert right after making a purchase to eliminate this risk but then you'll lose the interest from holding the cash in the offset and its more of a hassle to remember to do this each time.