The efforts the Garamantes went to so they could sustain agriculture reminds me of the Sabaeans in modern day Yemen and their great dam at Marib and extensive irrigation. Tough places to eek out a living.
From a cost and energy per cubic meter of delivered fresh water there’s a break over point where it becomes cheaper and less energy intensive to desalination of seawater and put it inland vs pump it up from an aquifer and transfer that water via an aquaduct. From a pure energy stand point for water used at the wellhead vs at the gate of a desal plant the breaking point is about 1500 meters of depth using multistage submersible lift pumps vs reverse osmosis pumps and membranes. California is in this position they have massive amounts of deep ground water but they have the Pacific ocean down the whole length of that state for the cities on the coast its almost twice as energy efficient to desal vs pump up from 1000+ M and transport that water over a hundred kilometers or more. For an oil rich nation like the middle east it comes down to access to technology and how much oil you are willing to burn to get water.