A lot of your route is already mapped. There are some unknowns from Cape Girardeau to Nashville.

Take the Mississippi river trail to Cape Girardeau Mo, catch the Great Rivers South route to somewhere between Dover and Waverly TN and go East to Nashville. Get to Chattanooga using this route https://ridewithgps.com/routes/29346270 and from Chattanooga follow this route to Atlanta https://ridewithgps.com/routes/10899044 .

First I search Adventure Cycling Association and US Bike Routes. I then search strava and ridewithgps for someone else's route to modify but if that doesn't work I autoroute my start and end points either using strava and ridewithgps. I follow the map and check the strava and ridewithgps heat maps and alter the route as needed. I then check for construction, road closures, and road conditions (street view) using Google Maps.

I'm riding from Indianapolis to St. Louis, down to Gape Girardeau and then across to USBR 35 from Louisville to Indianapolis in the spring.