Answer: Based on your description of the location, the growing conditions are terribly hot and dry in addition to being in harsh strong full sun. Most plants just will not survive there, particularly since the soil is shallow. Larger plants will have a proportionately larger root system, so if there is no space for the roots to grow then the planting spot will not support a larger plant. Since the Santolina did well, I would suggest you try Sedums as these will provide a nice textural contrast in addition to blooms. You might also experiment with the different thymes and possibly also some dianthus. If it is very sheltered in winter you may also be able to grow some of the hardier rosemaries such as "Hill Hardy" or "Salem" or "Arp."
Barberry shrubs might survive there and are available in red and yellow foliage forms in addition to green, however they are thorny and may not be a good choice if the space is tight. For more height, you might be able to grow a rose of Sharon or Hibiscus syriacus there, although I am not fully confident that there is enough root space for this deep rooted and large plant.
You might be able to grow a vine such as Parthenocissus tricuspidata or Virginia creeper on the wall. This will provide a nice softening effect on the space and help reduce heat reflecting off of it, however it is a very aggressive plant and would likely crowd out your other plantings in a few years.
You could also consider showpiece tropicals such as hibiscus and bougainvillea and lantana standards in large, self watering containers (bring these indoors each winter.) Tall growing cannas might thrive in the heat and sun, too, if you have a large container with rich soil and ample water.
Your local professional nursery staff may have additional -- or other -- suggestions based on a more detailed understanding of the planting site and your design goals. Another consideration might be to cover the area with a pergola and provide some shade. This would expand your plant palette somewhat.
I hope this gives you some ideas!
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