Between Austin and San Antonio is so developed that it's considered by many to be a single "metro" area, DFW-style. There's very little not developed directly between the two.
This was 15 years ago. Maybe it was less developed then, or maybe my memory conflated that drive with another part of that trip.
I drove that way in 2024 for the solar eclipse. Some parts of that route struck me as a bit exurb-ish and spread-out, I wouldn't call it a single metro area, but there were definitely people living there. And it was way too green to be called a desert; I've driven through actual deserts in southern CA and nowhere that I saw in that part of Texas was anywhere near that dry (I guess you have to go further west to get to actual Texas desert, which we didn't do on that trip).