You could, of course, order the relevant ACA Southern Tier sections for Texas. Don't have to buy them all.
The book
Best Bike Rides in Texas by Andy White chronicles a few dozen round about rides, many close enough together that you could ride from one to the next.
Other than ACA and the book, it's probably a DIY proposition. You could research the journals
here for ideas. I ride in Texas a lot and rarely run into any really bad roads for cycling. Stick to US highways and you'll always have a shoulder. Lots of the state highways also have shoulders or long sections of 4 lanes. Chip seal surfaces are the biggest problem. TDOT loves that awful stuff.
For just a long, smooth, cross Texas route, US 380 in North Texas is great. Conveniently spaced services, mostly flat, light traffic except for Denton/McKinney section. Great historical state park at Jacksboro, and nice city parks in small towns along the route where you can pitch a tent overnight.