Fertilizer burn maybe?

I'm growing full sun, but my area is much more humid than yours.

Not Sunburn. Sunburn would show on the Caudex (girth of Plant) as a yellow/gold area which was facing the sun. They can take extreme sun, but the Hot heat and dry can cause sunburn. You sound like you are giving goo condition to safeguard it.

Leaves at the top look to be showing fertilizer burn (yellow and biege crispy leaf edge). Make sure to flush it every once in awhile w plain water.

Rick