I routinely book through European consolidators (there are quite a few of them) for US car rentals... because they almost invariably offer "all inclusive" pricing. CDW/LDW. Liability insurance etc. What is left is the excess, and a few minor benefits, which your travel insurance is good for...(.if indeed the rate doesn't include zero excess)
Most of those websites IME base the quote on your stated nationality/country of residence so I have never been concerned about that side of it. Yes, it is true that windscreens/tires and underbody damage are not necessarily covered. I don't take the extra coverage there..I suppose you could say I "self insure" for those... and in nearly 60 rentals over there I haven't had to pay up... A windscreen could of course just be bad luck... but tires and underbody damage seem to me to be pretty unlikely - if driving normally on proper roads...
Why are the prices different? Why are airfares different from AAA to BBB compared to BBB to AAA? Market forces at work? If you get some benefit from booking through AVIS then well and good.... but (and this depends very much on what vehicle/where/when you are actually renting) even that AVIS UK quote looks a little high to me.... You may of course be looking for something larger than I ever need...
P.S. I would just about guarantee your Aussie issued credit card will provide ZERO insurance benefit on car rentals. That would however only be significant if you did what some do...book the very low rates available within the US......meant for Americans.. which do NOT include any Insurance or damage waivers... I can't think of many things more frightening…. ALL of the avenues you are looking at so far will cover you adequately....