Lawns with Constraints

Our back yard is home to an enormous pecan tree. An arborist told us it's certainly over 100 years old. It's a beautiful centerpiece for the space, giving us a huge bounty of pecans last year, and, more importantly, much-needed shade in the hot months here.
I'd like to have a nice, barefoot-playable area in the back yard for our kid and friends. The go-to eco-friendly native grass in these parts is Buffalograss. Once it's established, you don't need to water it. Perfect. But it loves lots of sunlight. Not good for growing underneath a massive tree.
When we moved in, the yard was overgrown with annual grasses (they get huge, then die), both native and not, as well as Queen Anne's lace and thistles. But last fall, while mowing the weeds, I noticed a few small patches of St. Augustine grass; what seemed to be the remnants of a past lawn.

St. Augustine is known as a water-hungry turf grass, commonly used in lawns around Texas. Aesthetically, I find it quite pleasing. It's the grass that I grew up with in my front yard. My dad treasured his St. Augustine and worked a lot at maintaining it.
But, I don't want to use city water to maintain my yard. Austin is a city often stricken with drought. Water should not be wasted in central Texas.
So, I'm trying to see what I can do without watering. It may be a fool's errand to try growing St. Augustine without watering it, but we'll see. From what I've read, its water needs are much reduced when it's growing in the shade, and it seems to be one of the few grasses around here than can do well in the shade.
For the past few months I've been selectively pulling the weeds around the extant patches of St. Augustine, and it has flourished, spreading to many times its previous size. We've also had a good bit of rain in the spring and even into June, which seems more than average. So, come July and August, maybe all of this new growth will die. We'll see. I'm hoping I can have my eco-green cake and eat it too.
Here's what that above-pictured patch from last fall looks like now:

And here are a few of the other spots in the yard:


I'm also trying to cultivate the native horseherb in the yard too (pictured below on the right):

That's all for now. A post about a lawn. I'm really enjoying the process: taking something that was there, and just putting in the effort by hand to see what can come out of it. If it all dies, I'll see what else I can grow. Also, expect a future post about adding some sort of rainwater collection system...