I'm also in a hot summer, mild winter climate, though just a little cooler than you with winter min down to -5C
Japanese maples need plenty of water to reduce leaf burn in summer but it can get to the stage where they just cannot transport enough to the leaves even when the roots are damp so some leaf cooling is needed in hot climates. The peat and coco fiber should be good for you to retain plenty of water but will still need really good watering. The key is to get the pot properly wet to start each day. Dry soil is hard to wet so I water each pot 2 or 3 times - water all the pots then go back and water again, then maybe water them all a third time. The first water soaks in a little way but most just runs through the pot and out the bottom. Resting for a few minutes allows the water to soak in a little and when you water a second time the new water will penetrate further and wet the particles better.
You may need to be careful that the roots don't stay too wet in winter. Sometimes fungal infections can affect the roots of Japanese maples when they are constantly wet and cold. keep pots up off the ground, especially in winter.
I also use humidity trays to help trees cope with hot, dry weather. Fill a tray or tub with stones and place the bonsai on to of the stones. When you water the tray fills with water but the pot is not sitting in water. During the heat the water in the tray evaporates and helps cool the tree and reduces the amount of water it needs to draw up from the soil. Some roots will also grow out of the pot into the wet gravel so the tree will have access to extra water. This is a good strategy for maples and smaller pots in hot dry areas.
Protection from hot wind is also important. you should try to get a screen on the hot windy side to reduce the impact of hot winds on the trees.
Overhead shade will also help but not too much or you end up with long, soft shoots and unhealthy trees. I've used 35% shadecloth over my growing area that goes up in December when the weather starts to get hot. I take it down again at the end of the hot weather so the trees have full sun for the rest of the year and better cold in winter.
Many of my maples still end up with some leaf damage. I can defoliate in late summer so there is a fresh crop of undamaged leaves for autumn colour change.
It is possible to grow maples in warmer climates. They may not grow as well as in a cooler winter area but still worth trying. Note that trident maple is much hardier and is even better and easier to manage as bonsai.
You've totally earned my respects! I haven't found this much useful information about Japanese maple-caring in hot climates! I do use a shade mesh of about 50/50 from June to October (Here in the northern emisphere, basically when daily maximum are over 25ºC)
About that wind protection, I have some weeping willows that produce a lot of foliage and branches each year, and given that they need no special treatment but water and more water, I'll use them for now to protect (kind of) the Japanese maples.
Watering won't be a problem, although I use tap water, it's really not that high in minerals, as I watered all my plants with that tap water and they've never presented any clorosis nor any other kind of symptome. I do the same when watering my elder tree, poplars and willows; I water once, then wait, then water again.
I'll try the trick with the gravel and tray, as I noticed in a small Pine (Pinus Eldarica) Seedling I was growing in a plastic cup, the bottom part where the drainage wholes are found, I normally fill that part with pure red lava gravel, and so I saw that roots that reach that part are like 2 to 3 times larger than those in the upper part. If I lived in a cooler-more humid climate, I would use a more drained substrate as it definitively makes roots healthier and grow vigorous.
You defoliate the whole tree or just the damaged leaves? I've defoliated some trees before for better autumn color, but never to a Japanese maples.
Trident maple is still out of my reach, can't find it in any local store and it's pretty expensive in any of the on-line stores I usually buy in.
This is part of my terrace in late november/ early december. Note that all these have been there throughout all the summer.
As you can see, wind gets everywhere as well as sun.