Yes, a grey water system, even to capture some of it would make a huge difference to what you could grow.
And yes re nurse plants. I did a service trip with the Sierra Club in Joshua Tree National Park one time. It's high desert, so cooler, but also about 4" of annual rainfall. Joshua trees cope with this by making huge root systems, like 70 feet in diameter, which collect and store water from all that area. One of the things we did (along with eradicating invasive exotics - read weeding) was to "plant" dead shrubs. Just sticking dead dried out brush in the ground. The point of this was that it provides a little bit of shade AND funnels water (rain water or dew, condensation) down to the ground, like a collector.
You could see where people had done that in previous years. A little green oasis was forming around each one, even though we planted nothing but the dead brush.
Someone wrote in here recently from some desert place who was thinking about doing a sunken garden. It's the opposite of the raised beds the rest of us do to promote drainage. By sinking the garden, it helps collect the water as well as create shade on the plants. You can also make dry creekbeds lined with stone, which will channel water when it does come and give the illusion of a creek the rest of the time:
https://www.coloradolandscapedesign.com/ ... ek-Bed.jpg
Of course mulch everything and add lots of compost/ organic materials to your soil, to help it hold moisture better. Plant drought tolerant stuff. If you are irrigating, only use drip irrigation under the mulch.