There are several different species of mycorrhizal fungi, each offering specific benefits. The better products contain several for a good all around solution. With so many product choices, I would like to use something others have had success with rather than experimenting.
Last year's record heat and drought really stressed my leyland cypress trees. I've lost one, all the evergreen shrubbery in front, as well as several large bushes. After digging them out, I found the roots were mainly between the dirt and the mulch with the dirt being almost lifeless: no roots, earth worms, organic matter, nothing. Whoever did the original landscape didn't improve the dirt at all before planting.
You are welcome to the links.
It sounds like you are on the right path to solving your problem.
When I first came to Arizona, in Wellton, I met a Lady who had lived there over 50 years.
There we also have layers of pure sand and pure clay.
When I went to plant trees, she taught me to dig a big hole and make it deep and throw the tin cans from food in it, then fill in with good soil and compost and the tree.
The reason for the tin cans, was water reservoirs to hold water and allow the roots to find it. The cans will rot and return to the soil in a few years and the roots will grow right on through them and down.
I tried it and it works.
When I planted any plants, I made the hole larger than needed and filled it with good compost or soil/compost mixture.
The tin can hole cannot be too big, 4 by 4 foot or larger for a large tree, we all kept a big hole for cans and sooner or later found a tree to go in it.