We did 3 weeks from Vancouver September before last. The best hire car deal then, pretty much usually lately (Stuttgart as recently as last month was the same), is via the BA Avis tie in, though you don't need to be a BA passenger to use it. I'm on the road atm, but Avis.com/BA might be it.
My relatively extensive experience of hiring cars in North America has always been hugely in favour of completing the hire from UK. the Americans seem to pay far more than us and upselling at the airport always seems to be a win for the hire company. Other folk may have had different experience, but I guess it rather depends on the UK deal as to whether the upselling seems cheap.
Convertibles can be a bit of a lottery, depending on start location - I've had good deals a few times, but sometimes the premium has been outrageous and sadly not worth it. I fully see the draw however. Out first rag top hire was from LA a dozen or so years ago and the experience in Sequoia NP and Yosemite NP, with snow everywhere, fleece and hat on, heater on full, meant our second car at home has been a convertible ever since! The roof is like an umbrella - it goes up only in the rain as opposed to down only in the sun.
Do go to Questor and consider their rental car excess insurance. I use them every time - in fact it's usually economic to have a years cover for any reasonable length trip. It'll save you a bundle over taking out all the various hire company excess covers and damage waivers.
What an awesome trip and a very good time to do it! Enjoy.