You could do the Southern TIer - but it is the least scenic of any of the ACA cross-country routes.
My first X-USA was from Astoria, Oregon to North Carolina - starting Sept. 1.
Fall trips are fantastic - but -
The days are getting shorter and shorter - esp. after Oct 15.
You can get caught by early snows in the Rockies.
Campgrounds and services in park areas close in the fall.
If you do a fall trip - I definitely suggest west-to-east.
Also, it makes sense to do a diagonal - since temperatures are dropping.
Sept 1 - Oregon start?
Sept 15 - SFO start?
Oct 1 - Ventura start?
If you did Sept 15, could catch Yosemite and parts of the Western Express -
then swing down and nop the southern tip of the Rockies at Taos.
This would put you safey out in the Great Plains in early Oct. - Indian summer.
Perhaps aiming for the Georgia coast - Savannah - or better yet, Sapelo Island.