Sounds like a fun trip. A few tips:
The Alum Cave Trail in Smoky Mountains National Park is a nice hike. It's on Hwy 441, which is a nice scenic drive.
Not booking hotels ahead of time is tricky. Yes, it will give you more flexibility, but you also could run into issues where prices are very expensive or everything is booked up, specifically on Memorial Day weekend. Booking ahead in the big cities (DC and Atlanta) as you've planned is a very good idea. You might also consider using AirBnB, which has some unique options.
If you have extra time, considering driving off the Interstates. The Interstates (I-90, I-40, etc) are by far the fastest route, but you'll miss a lot of the scenery and local flavor. You can check this list for scenic drives that might be near your route. Stick to the interstates if you're just looking to drive fast and make good time.
Don't forget there are 4 alpine coasters near Dollywood if you're interested in those.
I have plenty of advice for Atlanta, especially things to do and local places to eat. Feel free to send me a direct message if you want any tips for this city. This might be a particularly good place to buy clothing since it's your last stop, and there's everything from cheap outlets (Outlet Shoppes at Atlanta, Tanger Outlets, North Georgia Premium Outlets) to huge malls (Lenox Square, Phipps Plaza) to unique shopping areas (Ponce City Market, Atlantic Station, Little Five Points, Westside Provisions, Virginia-Highland).