Goldenball leadtree (leucaena retusa) or desert willow (Chilopsis linearis) would fit the space and provide lighter shade than some of the other options for your under-plantings. These and other good options are included in the plant guide Austin keeps on their website.

Magnolias are beautiful trees, but drop a lot of their tough leaves; something to consider if you're planning to plant around it. The one on our property line finally motivated us to trade the leaf blower in for a leaf vacuum.

Vitex pruned as a tree might look nice there, and they grow rather quickly.

I also like Rusty Blackhaw Viburnum, but it might be difficult to find in Austin. They're native on my property in east Texas. Pretty spring blooms, berries that ripen blue, and gorgeous fall color.

Lin

Along with vitex, you could prune a wax myrtle into tree form. But I love crape myrtles. Magnolias are extremely messy and it is sometimes difficult to grow under them. If you get a redbud be sure it's the Texas variety. We planted an Oklahoma redbud that cratered quickly and never bloomed well.

Magnolias are picky about their soils and love the water. Austin is flirting with stage 3 drought so I would think of trees that are fairly drought resistant. Redbuds, Mexican plum, Wild olive, Escarpement Black Cherry, Texas Ebony, Anacua,

Crepe Myrtles and Magnolias don't do very well in Austin.

Try a Yaupon Holly or Redbud or Mexican plum.
A Mexican plum will look like a mini Bradford.
Personally, I'd go with a Redbud, tho.