Replying to bump up your post. Maybe it'll get some attn.

If they were mine, the first thing I'd do is take them out of the pot, out of that mix, and see what's going on with the soil -- and the roots. You describe it as "with potting soil mixed with sand just like where they came from." To me, that may be half the problem.

I don't grow Stepelia, but Huernia. My best growth has come from potting mixes with NO soil -- usually just pumice, or a combo of pumice, Turface and grit. There is A LOT of info about it on this forum under "gritty mix." I have also successfully used a blend of 50% C/S soil and 50% pumice.

I grew mine in direct sun for awhile -- they didn't do well. Their vibrant green color yellowed tremendously. I moved them to bright light (but slightly shaded) and they flourished. They grew long, flowery, and some of their former color returned. One flourished so much that it had to become a hanging plant, so I gave it to a neighbor who specializes in hanging and draping plants.

I've grown Stapelia but gave up because they were often attacked by mealy bugs. When they get a lot of sunlight they turn purple and have a lot of branches of 3 to 5 inches I think. They seemed to grow and flower better when they looked like they were under potted a bit. Some things I noticed about your Stapelia: the branches are quite long without a lot of branching and green and the pot looks over sized. So, I'd suggest a smaller pot and gradually giving it more sunlight but I don't know how well that would work for you in Yuma.

My huernia. Gifted to me by a forum member. The 3 draping arms are the originals that were bright green and draping when I got them. All other growth happened in the 3-4 years I’ve had it. Not sure if they will eventually drape too as the get longer.

If your moisture gauge says they are dry and the stems are drooping, doesn't that indicate they are in need of watering? The stems look like they are getting sufficient sun and look healthy. They tend to go reddish if stressed with too much light or too little water. I agree that your plant looks overpotted.

No sand in the mix makes it a better mix already.