No sign of gophers that I can see from you photo, is that right?

Does it droop during the day then perk up as the sun goes down? If it droops then recovers with evening cool, it's the heat. If it's not recovering at night--not good. Any chance it is over-watered? They like excellent drainage.

They are generally coastal plants and do not like it super hot, but if you've had it for 5 years, you'd think it would have adjusted.

You said you've been limbing it up to be a tree--that might be a factor. Roots less shaded than before? Trunk needs projection from sun? Just tossing out possibilities.

I do not think the heat is the problem, something is going on with the roots that is preventing the tree from using the water and nutrients in the soil. Not a lack of water in the soil, with a lush lawn as shown, more likely too much water. I would check the soil for excess moisture. Your tree should also not be strapped to a stake. Growing a tree with a trunk that cannot move with the wind, leads to a poor root system, lacking support, even more so in a lawn which discourages deep rooting. Al

Don't know, agree maybe too much water, root rot? May want to take your picture and a branch to a garden center and see what they say. Good luck.