UPDATE:

I tried most of the advice by the posters above, and kept experimenting myself.

I would term my final best practice is to create "the circle of death".

I now spray glyphosate (old version of RoundUp) in the early spring after emergence of the cool season grasses and weeds. About a week later, I run the weedeater and add a fresh layer of mulch. I then spray another round of glyphosate in June when the warm season plants have emerged.

I now have circles about 6-7' in diameter around most of my recent tree plantings and have not lost a tree to varmints for at least two years. I can even see lines of pocket gopher mounds or armadillo rooting holes that hit the circle and then go off in another direction.

The only caveat is that we have had good summer rains the last two years, so my "circles of death" are always surrounded by greener areas for foraging creatures. I do not know if this method will work during a summer drought when all of the growing plants go dormant EXCEPT for my small irrigated areas with lush tree roots. In that case, I may spray glyphosate more often to avoid anything green but the trees themselves.