Buccees is great, but they're not inside the beltway. The bathroom by the layaway department (very back of store) in Walmart is usually clean. I'm in outside sales and that's usually my go to place. HEBs are usually a safe bet too, but I feel like people judge me more there when I leave without buying anything. Electronics stores are usually clean too, because they see little traffic during the work day. Stripes can be found inside of beltway, they do a decent job, although some are gross. Starbucks are clean most of the time, but are usually just 1 person restrooms... So it can be awkward leaving if you take a 30 minute deuce.
Once used this restroom in Sulphur Springs, TX OUTSIDE Spoiler INSIDE Spoiler You can see everything outside around you on all four sides .