I wouldn't expect Thuja plicata to do well there.

Seems Thuja occidentalis can be hit or miss in DFW. There are quite a few beautiful survivors in our area developed ~50 years ago on the north side of Dallas... and others that have struggled, declined, and departed. I'd certainly try one if it's a favorite. Root rot can be a problem, so good to find it a well draining spot in our clay.

Learned of the sterile 'Sunshine' ligustrum at the Dallas Arboretum. Evergreen with yellow/gold foliage given enough sun. Tops out at 5 or 6 feet, so may not get tall enough for your spot.

I can amend the soil, but any ideas on how deep and wide do these roots get? That is, how much soil will I need to amend?

Thanks for the tip on sunshine ligustrum!