I shade plants all the time with shade cloth since I'm on a south facing hillside. I have 50% on the propagation benches and some 70% on some container plants. It helps tremendously. That tan home depot stuff is around 70-80% iirc. My experience is that it can trap heat beneath it, however because of the cut in solar radiation the evaporotranspiration is greatly reduced anyway. I have also found that people greatly underestimate the amount of solar radiation we have in SoCal. I have had people tell me that certain orchids might not bloom under 50% but for me they bloom fine and actually show signs of having too much light.

Okay great - what level would you suggest for my veggie garden? Are you familiar with the row cover material at all?

I think you're right about underestimating the UV - I was wondering the other day if "full sun" should be climate specific - I was afraid my beds don't get enough hours of sun (only 5 hours...) but I can't really imagine 8 hours of this strong sunlight! The light in Massachusetts never felt like this...;) Maybe there you need 8 hours but here less will work? I can't find anything to confirm this but it sure feels true...

I've been shading my baby squashes, etc. with an umbrella in late morning when they seem to wilt. The bigger plants seem to do alright; it's just the little ones that have a problem.

Summer vegetables once beyond the juvenile, are quite able to take the summer heat, provided they are properly hydrated. They may show some wilt during the heat of the day, especially when the heat increase is very sudden, but they will adjust. Al

The raised beds up against the house is a double whammy on hot days for anything young and/or cool season and later in the year could be hard on anything.

Not saying shade cloth isn't a valuable tool, I use it often year round since our weather fluctuates so crazily. In the winter on the seed starting tables, spring summer to grow greens later and shade some tomatoes and peppers.

I usually use 30-40%. Air flow is big, and ground level plantings or even trenches help with soil temp and plant level air temp and humidity.

The trellis along the wall is indeed a double whammy - there is actually 24" or so of room between the wall and the beds - hard to see in the photo, but there is air flow through there at least. Our yard is small - maybe 20' x 25' - most of which is shaded by a giant ancient orange tree so options are a bit limited. This is our first year here so we are on the steep part of the learning curve ....

I'm going to rig a better structure to attach the shade cloth to and see which fabric seems to work better...

We all had/ have to learn. I find trying that solving one problem often brings on a new set of them!

The practice you are doing will make you a very good gardener in your setting.

A California vegetable growing forum would be a good thing.

The other nice thing about shade cloth is that is does break the wind. When you live on a south facing hillside that routinely gets10-20 mph sustained winds like we do, that is a big plus.

One example of how fixing one problem led to another is the use of drip irrigation in single rows.

Plants started (or existing) while there is some rainfall will grow roots outside of the normal drip zone due to the broader saturation by nature. Then, when rain stops the roots no longer get water out there from the drip and wilt or die outright or give mis-shappen fruit at low yields. I see this most right now with the zucchinis that I had planted,but I am sure it could happen with anything.

So wide beds work best ,with small rooted vegetables planted on the out side of beds and larger rooted plants on the inside. Like tomatoes down the middle with basil to one side, for instance. Or cabbage down the middle with beets on the outside in winter. Doesn't so much apply where rainfall is more normal year round or at least for the duration of a crop.

I use shade cloth but will not put it up this early in the year, we'll cool down tomorrow for a while so shade will probably go up around late June. I rig mine on top of the bamboo framing that covers the entire raised bed garden (holds the bird mesh), it's about 8 feet high. So the shade has a lot of airflow under it and also does not shade fully all day, as the sun moves different beds are more or less shaded. Plus it shades ME when I'm out there in the heat. I use some motion picture scrim cloth that husband had around, probably 40-50% shading. When I saw the OP's photo my first thought was you need to sink a couple of poles to hold the low side up so the plants get airflow, in really hot weather the accumulated heat at the low side may be harmful to plants. Re squashes seems the leaves always wilt in the heat of day but perk up in the late afternoon when it starts to cool down.