Sounds like a good plan to me. Having driven rental Mustangs in the US I’d be avoiding them but I totally realise why it’s important.
If you’re intent on Route 66 try and plan some time around Sedona. It’s the best part of it imo.
IfI was doing it I’d be watching every episode of Diners, Drive ins and Dives and working out a plan based on that. The foie gras and chips place in Chicago sounds like an ideal first stop to me.
I just used booking and picked my next hotel based while staying in the last one. Price and availability is worth considering near national parks but no problem anywhere else. You don’t need to stay in ok motels when for the same money there will be a hotel with breakfast thrown in.
I’d be looking at finding a route viaMt Rushmore then Colorado.
Lastly, watch some geology programs on telly as it is so open and visible in places and you probably don’t remember if from school.