Much better hills are inside the Palo Duro State Park outside Amarillo.
San Antonio here. Green Mountain Road, just north of Hwy 1604, is a good workout. Not killer steep, but a good steady even ramp of 200' over a mile. A better workout is the 4 miles of Smithson Valley Road between Bulverde and Hwy1863. A steady slope with a nice 50-100' ramp about every mile. I tell myself it's like doing intervals.
Just saw this thread. As the OP noted.
I did the Ft. Davis cyclefest (75 mies) this year along with the Ft. Davis Hammerfest the following day....which is a seven mile ride with almost 2,000 feet elevation gain. Other than the steepness of those climbs the elevation got to me. 5,000+ to almost 7,000 feet on the day two climb.
Since I ride and live near the gulf coast that is a big difference in the elevation. The Ft. Davis rides are awesome if you can get out there in September for the event.
I also rode ranch road 337 between Vanderpool and Leakey Tx this year. Those hills are grades approaching 15-17% in parts. Very tough climbs but not as long as Ft. Davis. This area was also mentioned by the OP.