Thanks all for the... forthright feedback, it's appreciated :) Especially around Yellowstone - looks like the average travel time my map planner gives me might not apply there!
Shopping itself to be fair is not a major concern, the better term would probably be 'quiet days' i.e. days without a whole lot of travel pre-planned so we can just chill out. But I think given the amount of ground we're trying to cover, you're probably right about weighting too much on those. Ideally, we'd also prefer not to be putting down huge miles in a day more often than necessary.
I'll do some more working and come back with Plan C :)