I designed a large commercial job many years ago that had a triangular island in the middle of the parking lot with no possible way to get irrigation to it. I filled the space with 3 different varieties of agave (around 40 plants) and around two dozen really big barrel cactus.

Zero maintenance, no one stole the plants, and everything bloomed once in a while.

I've found that one of the biggest problems planting under a crape myrtle is their dense root systems. What size is your tree? Does it provide some shade to the area in the middle of the day? If it does that will open the area up to a wider variety of plants.

Some plants you could try are native wild petunias (Ruellia caroliniensis), sun loving bromeliads, native false garlic (Nothoscordum bivalve), rain lilies, blue plumbago, crinums, and native monarda (Monarda punctata). All of these are plants that I have growing under my tree crape myrtle. Only half of the bed is watered by the irrigation system, the other half relies primarily on the rain.

Dwarf Indian Hawthorne. I planted them under my crepe in full sun and they are doing great! They are 2-3 feet tall full grown, they are evergreen and they bloom pretty white flowers in the spring.

purple trailing lantana
; ]

Thank you so much for all the suggestions! I'm going to sit down this afternoon and look them all up and make my pick.