My tomato plants are smaller and the yield sparse. Zucchinis only
occasionally produce - usually shrivel before getting any size to them.

Time for a soil test... I'm suspicious of too much sodium (I lived in S.
Cal 30 years ago, and the water was pretty saline!). I'm also suspicious
that you might have had more sun on your garden plot 20 years ago than you
do now.

Yes, less sun, more tree. I wonder if morning sun, like 8:00am - 12pm
is better than afternoon sun, 12pm - 4? I have a choice between two
areas, west side of the tree or east. Now it's on the pm side.

I think there's the problem, right there... "full sun" (for veggies) is usually
6 hrs of direct sunlight per day.

As to AM or PM:
Probably no real difference in your climate, unless you're trying to get
a crop of warm season veggies in the winter (you'll want the afternoon sun
then) or veggies happier with cooler climates in the summer (you'll want
morning sun, afternoon shade).

Still probably worth a soil test... salinization of non-drip irrigated soils
in S. Cal is a problem.

Kay