DITTO RBG on selecting region specific, drought tolerant varieties of vegetables.
SIL waters her table top boxes with a watering can. Watering only the base of the plants. Her "mile high" climate is arid, has persistent winds and high temperatures in the summer can be higher than in the deep south. SIL is a scientist (as in 2 PHD's and multiple certifications) and an avid preservationist. She is very pleased with low water usage and ease of working her vegetable garden.
She does use Mel's Mix. She maintains a compost bin and purchases peat and horticultural vermiculite when needed.
DO get into composting.
I had a kind of crazy thought.
Typical SFG boxes are 6" - 8" deep. The bottom is lined with contractor grade, woven, landscape cloth (not the cheap felt like stuff you find at big box stores)
Dig down no more than 1" into your native clay soil. Line and build your boxes deeper - like 12". You will create a "water pan" beneath your boxes. Not so deep that it retains huge amounts of water.
Crazy but that is what you get when a gardener is presented with a problem!
I am really looking forward to hearing more from you. Keep us updated on your decisions and your progress.