Is it cheaper to book hire cars before I go tho the US

#! and #10, I always take travel insurance that includes car hire cover - not just cover but the excess as well to about $4000

You've received good advice thus far. Renting at off-airport locations is a good money saving tip, and the upgrade should be free if the class that you've reserved is out of stock. And car rentals can generally be reserved without any deposit, and can be canceled without any penalty, so reserving well in advance is generally not necessary.

>>>I always take travel insurance that includes car hire cover - not just cover but the excess as well to about $4000

CDW coverage doesn't work like that in the US. If you buy CDW for US rentals, there is no "deductible" (what you call "excess" in Australia.) This differs from Australia, where CDW coverage may provide just a partial waiver.