A couple of notes -

Be prepared for extremely hot and humid weather in Washington DC and the east coast in general in August. I would do the east coast by train and save the driving for the west, where both the weather and options will be (IMO) far better.

Driving from Seattle to California is a very popular trip. I would look at a conventional car or SUV between Seattle and San Francisco; if you really must drive a Mustang (I assume a convertible one) then pick it up in California, otherwise the one-way hire cost will be astronomical. (Bad enough as it is, it's awesome when you throw in a specialty vehicle cost.) And be aware that a convertible in the desert in August is a quick one-way trip to the emergency hospital if you're not careful. Not to mention a rather tired cliche - just about every Mustang you see on the road in coastal California in the summer is being driven by an overseas visitor.

Yosemite is glorious, but by August/September some of the waterfalls (one of the real draws) are dry or nearly so. I'd stick to the coast from Seattle to SF, as it's (a) gorgeous, (b) cool, and (c) offers the redwoods, usually stated by overseas visitors to be the highlight of the trip.