Those spheres can grow and shift over time. Look at cities like Waco and Laredo, eventually they will be their own economic spheres rather than simply sphere adjacent. The cities and suburbs will continue to grow, but if you want to help small towns Texas really should be pushing for more Federal interstate dollars.
While we have the most miles of interstate in the US, the majority of those are taken up by just a handful of routes. As far as number of interstate highways Texas only has 16 while California has 26 connecting early every major city to each other. Interstate 14 is going to be very beneficial for many of those communities West of I35, and I69 will help connect South Texas>Corpus>Houston>Deep East Texas>Texarkana but there are clearly other areas of opportunities in this state that could benefit from this type of expanded infrastructure.
Austin>Houston, DFW>WF>Amarillo(w/ a Spur to Lubbock), Deep East Texas>Central Texas (mirror of IH14).
Just look at a map and see all the small town that this could benefit. Clearly this is a double edge sword as they could just turn into glorified suburbs down the road, but if we are worried about these towns truly dying, this is a great way to increase population.