If you guys take US-59 from Texarkana to Marshall, TX-43 to Henderson, US-79 to Jewett, FM39 to North Zulch and then TX-21 to San Marcos where it meets I-35. You will save about 100 miles, 1 hour and go about 8 miles from my house on your way down. You will also go within about 1 mile of Fuzzy, Pistolnut, 40Grit, Nofearnohope and probably a few others.
The I-30 and I-35 route you listed is high speed and easy. It takes me 5-6 hours no matter what way I go from my house to Texarkana and I go your proposed route when I have a trailer just because I can set the cruise and just go without thinking or worry about local speed zones.
However, if you wanted to get the GSA truck so you can leave direct from SA. This might be a good option. If you were planning on me being a maintenance staging area for both trucks, which you are more than welcome to do, then go your way.