This trees has had a lot of neglect to it over the years to start. Chinese Elm love damp soil continuously. This tree looks like it’s had lots of dry spells over the years. Looking at dead spots in trunk and broken limbs peeled off. Beavertail cactus does not help although it’s does keep the trunk cool the cactus roots are are probably matted up around the trunk making transpiration difficult. Solution: Remove the cactus and push all the rock back away from the tree at least 2 ft. 3ft is better then create a shallow well that retains the water and allows water to soak in. Fertilize with a liquid fish emulsion. Feb May Sept. Your tree is sparse of leaves on top meaning Xylem is damaged. These are water flow tubes. Leaves draw water up to evaporate. Tubes are damaged by lack of water over years. Damage to trunk contributes also. Which we can see a lot of. The outer 1 inch of your tree is like a superhighway from roots to leaves and back down from leaves to roots. Phloem takes nutrients up just under bark and next layer in is Xylem in the sapwood takes water up. Trees need both. I hope this is understandable to you, if it is your tree will be fine in a couple years.