I just got back from the south side of the park to just a few miles north of where you are. I took I-75 north to I-675->I-285->I-85 then I-985 to US 441 which goes thru the park to Gatlinburg. 441 is mostly 65 mph and 4 lane. I stopped south of the park. On the way back I left at 9 AM, stopped for breakfast, and then drove the rest of the way home and was there at 4:30. So a bit over 6 hours driving time subtracting one fuel stop and the meal stop. You would need an hour to 90 minutes to drive thru the park to Gatlinburg. I wasn't towing tho and could do 75 mph on the Interstates.
The fall colors are not near peak yet in the park, along the south end of the blue ridge the colors are nearer but not at the peak. It gets a bit cool in the evenings up there right now. It was coat gloves and hat weather at Clingman's dome at sunset when I was up there.