Welcome to the forum.

You don't say what you want to see along the way, except Mt Rushmore and Ben and Jerry's, but there is so much to see that I wouldn't even attempt all that in a month. You have to get off the interstate highways to see almost anything, so the mileage and times will be way longer than you have estimated.

I always try to keep my daily mileage to around 300 if I'm going to see the sights. There are some times when you can go up to 500 miles a day, but those are rare unless you've already been over the route and seen most of it before.

I'd suggest that you see what there is to visit, choose two or three good sights per day and see how much of that territory you can reasonably cover in a month. I think you will find that it won't take in the entire circle you designated. Choose quality over quantity, if you know what I mean.

Good luck in your planning and I hope you can see most of what you plan.