Compost is great, but there's never enough to go around, and sometimes you need soil amendments with a little more punch. The famous organic horticulturist, Alan Chadwick, taught that the best of the animal manures is cow, and I have found that it is a very good material for the soil. It needs to be composted for four or five months before use, but then it provides just the right balance of nitrogen and humus. If adding sand, it is far better to use sharp sand than the stuff that comes out of rivers. Wood chips rob nitrogen out of the soil while they are breaking down, and this process can take years. Peat moss is very acidic, so I would be very carful of adding much of that. Check out the link below to learn about Chadwick's Biodynamic French Intensive system Click on "Techniques" and then on the subject of interest. Here is a link that might be useful: Alan Chadwick

A big part of your problem is the raised bed format. Any soil that is raised up out of the surrounding median grade-level will warm/dry quicker. Even the top of natural hills display this phenomenon. Raised garden beds are notorious for this. With sandy soil, you've got a double-whammy. There really are very few reasons to not use/improve your native soil at your feet. Adding organic matter is key here. The roots of plants are the best way to inject carbon INTO the soil. Use cover crops whenever possible (I'm a huge fan of white clover, a nitrogen-fixer), and don't rip out the roots if you don't have to. I've had good results with hugelkultur as well. Otherwise continually add mulch to the surface, and let the bugs and worms pull it down. Also, where drought is a significant pressure, space your plants out 1.5-2 times normal; that extra space goes a long ways toward drought-tolerance for many plants (especially vegetables). S'all for now, good luck.

I grow potatoes in "fabric" grow bags, and you will read different opinions on how to do things. Growing potatoes is very easy, and it is hard to totally screw them up. I don't bury new green growth, the plants need sun, and it's not like the taller stalks will be ruined. You can add more soil whenever it seems to be appropriate to you. They should be at some depth, but the idea that if you keep adding soil higher and higher endlessly it will make them much more productive is an exaggeration.

Doesn't Bonsai Jack sell several different soil mixes? I have never been able to keep string of pearls alive. So I can't speak for them. But I grow string of turtles which is a peperomia (peperomia prostrata) and string of hearts in a chunky soil mix that is approximately 1/3 potting soil, 1/3 small orchid bark, 1/3 bit perlite.