Really going thru the small towns and avoiding the DFW Metroplex won't save much, if any time. For me, the worst part of the drive to Amarillo is on Hwy 287 north of Wichita Falls. Having to stop and slow down in all the small towns. If it were me:
287 to I-35--> south on I35 to Loop 820 in Ft Worth (about 3 miles) --> west on loop 820 to drop down to I-20 in Benbrook (about 15 miles) --> I-20 to Jackson MS--> S on Hwy 49 to Hattisburg MS which changes to Hwy 98 --> I10 in Mobile to Pensacola
As always, try to plan you drive thru the big cities during the non-peak hours. Usually that's AM and from about 4ish-6:30 PM the rest of the time around here