Wow--thanks for sharing that story, I can't even begin to imagine what went wrong or why they all died.
Sulcatas are very hardy creatures and I have one baby right now who's unstoppable when it comes to eating, roaming, and climbing.
Have you ever had your soil tested?
At my old house for some reason I could never get plants to grow on one side of my yard, I couldn't even get a weed to grow ha ha, I went through many drought tolerant plants that usually grow like crazy with minimal to no water and even being on a drip system I had no luck with that part of my yard. My next step was to get the soil tested since I was convinced there was something in the ground but I never got to that point.

But again you mentioned that your plants grow great and are cautious about what soil you use and no pesticides etc.

Hmmmm