It's a nice drive!I see a route from Custer to spearfish up hyway 385. Is that a nice drive, or is there a nicer or more scenic one to take (in an S90D)? I think a single charge from 100% will get me there.
You can also check out Mount Rushmore and/or Crazy Horse if large rock carvings interest you. And Deadwood is a funky old west cum casino town that is also on your path.
Wind Cave National Park would be pushing your range (assuming Custer SC isn't open yet and you intend to only charge both ways in Spearfish), but it's a pretty cool spot as well. If you can't do Wind Cave, maybe do a drive through of Badlands NP on your way west. I'm assuming since you're from Michigan you will be travelling I-90 through South Dakota.