Dutch white clover.

Some variety of wheatgrass that grows well in your area perhaps mixed with blue grama. These prairie type grasses look pretty good when seeded like a lawn.

Care to elaborate? Apart from Dutch white clover, you mention a variety of wheatgrasses, blue grama, prairie type grasses, it confuses me.

As to the mix of wheatgrasses, blue grama, and prairie grasses, I wonder why local commercial mixes here are always some kind of a mix of fescue, rye and KBG. I would have to buy special mixes online, not sure if that is a wise thing to do.

As to Dutch white clover, I can only find a local source of white clover seed without any mention whether it is of the Dutch variety or not. Although one of the uses mentioned on the clover seed package is for overseeding existing lawns, it makes me nonetheless worry about planting an invasive weed. How do I maintain the desired 50%-50% balance between clover and grass?

If it is sold for lawns, as opposed to farmers for forage, then white clover would be the Dutch white clover variety. Dutch white clover only grows to 5-6 inches high while the others might grow from 18 to 24 inches tall for livestock to eat.

You asked for low maintenance grasses, not turf type grasses. Traditional turf grass is fairly limited to certain well behaved varieties of a few select grasses. Those include fescue, rye, Kentucky bluegrass, and bentgrass in the cooler regions. Most of those have been hybridized for their behavior, but not all the badness has been bred out. For example even the fescues and Kentucky bluegrass will occasionally get unsightly seed heads. But in addition to turf type varieties there are forage and utility type grasses used in farms and ranches as forage and for soil building and retention. These are not generally used for lawns because they send up seed heads faster than the grass grows making them look shaggy in a lawn setting. Others do not grow very dense and will allow weeds in. But blue grama and the wheatgrasses are slightly different. They will grow dense when seeded densely and watered once every few weeks. These plants also do not send up seed heads until several weeks after being mowed. You might be able to get away with mowing once a month and possibly never watering.

Here is a picture of bpgreen's prairie type lawn in Salt Lake City. It consists of blue grama, western wheatgrass, canary wheatgrass, and strawberry clover.

You might be able to get the seed for this type of lawn at a local feed store or farm and ranch seed distributor. In San Antonio it would be Douglass King Seed, but you're a long way away.

If you plant clover I would not worry about any 50/50 rule. I would plant the clover and make it 100%. Clover looks great when it is 100%. It looks horrible when it is mixed into a lawn in clumps and patches.