Ok well first off if you have ever overwatered and the leaves turned yellow cause of this the leaves will never turn back green. I would just remove the yellow leaves. If your getting wilt during the day that could be a sign of not enough moisture in the soil or to much sun. Young plants that arent well established sometimes do this even with the proper moisture. To much acid in the soil can cause a plants leaves to turn yellow. If this is the case taking some epsom salt and put it around the base of the plant will help that part. No the epsom salt will not hurt the plant if thats not the case. As far as your raised bed 2 gallons of water can be enough or not enough. It really depends, for one any kind of plant in soil that has completely dried out may take more water than that. When a plant completely dries out the best way to water is by a slow long drip. If you just dumped the 2 gallons of water on the area it may just not of had time to soak in. If an area is wet or has been watered consistently and the ground is not completely dried out it will take less water to reach to the bottom. I really hope that made sense. As far as a soil moisture meter I personally have never used one, I tend to listen to my plants. If they want water they tell me if they wont less they tell me. I dont get to technical when gardening trial and error is my way to go. I try something if it doesnt work I try something else.

Can you take a large diameter length of pipe and do some "core samples"? Can you take strong, smooth stakes and drive them into the soil several places in each plant's root zone? Pull them out and touch them, smell them, see if they appear dry, damp or smell "soured" like a pot that's stopped draining. Until you can see what's happening under ground, you can't know the answer to that question. What you're seeing COULD be the beginning of black spot (yes, Knock Out CAN black spot). Or, it could be a water issue. How does the new growth look? Either under watering or over watering would affect the new, softer, sappier growth first. If it appears shriveled, it could be either. Too little water wouldn't support the new, soft growth tips and they would stop, either not forming or shriveling. Too much water, depending upon the drainage and water retention of your soil, could lead to root rot where the new growth would also stop, but appear like limp, wilted celery or lettuce that's been left too long in the crisper. If it's black spot or some other fungal attack, the new growth COULD still be looking decent, normal, what you expect it to look like. Extreme heat can cause foliage drop, too. As the plant experiences water stress, usually new growth halts and as it worsens, the plant reabsorbs the nutrients from the older foliage first, often causing them to take on "autumn foliage" type colors depending upon the genetics of the rose. To reduce the transpiration of water (water loss through sweating), the foliage begins to be shed. The logical conclusion is that all foliage is shed, leaving a bare plant which appears to just be sitting there. Once you can SEE and smell core samples (or the soil dug from areas in the root zone of the plants, but that disturbs more roots making water stress worse), or strong stakes shoved several places in the root zone of the plants, you won't be able to know if it seems too dry, too wet, smells "soured" from too much standing water too long. Determine what the soil conditions are below ground where the roots are and you have a better chance of figuring out what's going on with the plants. Kim

We don't mean to overwhelm you w/all this info. But there are certain basics required & proper mix (soil) is key. Sadly the mix you're thinking of using is awful for succulents. As you've been told sand is awful for them, so is 'regular' potting soil. Also particle size matters, it's best when materials are close to the same size, helps uniform drainage. What's shown in your link w/ pix of large & fine mixed together will impair drainage. The fine material will clog the air space in the mix & prevent the roots from getting oxygen, which the plant also needs along w/ light & water. Yes, many here will suggest 50/50 cactus & succulent mix combined in equal parts w/ perlite. This will give good drainage. So will 'gritty mix', as it's called here, which has several recipes to make your own which you can search for here. Pls. read up on succulents & learn that they have the ability to store water in their fat, fleshy leaves, so they don't need as much watering as many folks think. So when it comes to when to water, in succulents we say, when in doubt about watering, don't. Wait several more days then water. Using a sprayer (if you mean a mister) isn't good for succulents, best not to wet the leaves (encourages rot). Also best to water into the soil AROUND the plant, not on it. Sorry for all the details, but mix & drainage are super important in growing succulents; & usually the first correction folks need when they start coming here.

It does not resemble the classic viral, fungal, or bacterial diseases, although sometimes those present rather oddly so I could be wrong. It does resemble overwatering, or nutrient deficiency. Nutrient deficiency could be caused by overwatering. Soaking them daily just sounds like too much even considering the heat. The usual guideline is one inch per week, with some extra water in hot weather or extra if the plants are wilting from heat during the day. Have you checked the undersides of the leaves for insects? It doesn't look too much like insect damage but there could be juice-sucking bugs contributing to the damage.