Ask Mr. Smarty Plants

That is a fairly lengthy question, so let's deal with one thing at a time. First, the replacement of the St. Augustine (YAY!) grass with natives. That's nice that you are going to scrape it out with a sod cutter and give it away, but we really wish you wouldn't do too much to take care of it first. In fact, we'd really like to see it disposed of rather than passing it along to someone else, but we can understand why you wouldn't want to waste all the time, money and water that have gone into growing it. In terms of natural treatments, pulling the weeds out is about the best, least harmful and most effective way to reduce the amount of weeds. 

On the subject of erosion and putting in a grass and wildflower mix, here is an excerpt from a previous answer also dealing with erosion control in Texas:

"Grasses and sedges would be your best bets for a slope.  You don't say, but I am going to guess that you are having erosion problems down the slope.  The fibrous root system of the grasses are excellent for holding the soil in place.  Another benefit of grasses is that deer generally ignore grasses in favor of other 'browse' plants.  If you really are having erosion problems you might benefit from erosion-control blankets to stabilize your slope.  The erosion-control fabric works by slowing the runoff water and allowing sediment to fall out rather than be washed away. Seeds are sown under the erosion-control material and grow up through the matting when they germinate. Underneath the matting the roots of the plants growing through the erosion-control material anchor the soil to stop the erosion. If you use erosion-control blankets made of biodegrable material, they will eventually disappear leaving the plants to control the problem."

We would also suggest you read our How-To Articles on Native Lawns, Native Lawns: Buffalograss and Meadow Gardening, all of which should help you in your plans for a native lawn. 

When you refer to cut ants, we are assuming you refer to leaf cutter ants. Again, let us quote from a Mr. Smarty Plants previous answer:

"We went looking for leaf cutter ants, and found that it is an enormous problem. Like fire ants, these creatures are imported from Central and South America and have no natural enemies in our area. This very comprehensive site from Texas A&M AgriLIFE Extension, Leaf Cutting Ants, has a lot of information that you should read. The pest control people you have been talking to probably either don't know how to approach the problem or realize it is bigger than they can easily control.

To quote from the above site:

"Defoliation by leaf cutting ants can resemble damage produced by several other leaf chewing insects, particularly sawflies and leaf cutting bees. Trees defoliated by the leaf cutting ant usually are within sight of an ant nest and the ants themselves may be seen carrying leaves. Foraging trails will be littered with pieces of leaf tissue that can be traced to a feeder hole. Considerable damage to a plant can occur in a few hours. Small- to medium-sized trees can be stripped in one night. One researcher in South America estimated that a large leaf cutting ant colony harvested approximately 13,000 pounds of leaves over a 6-year period. This same colony excavated 802 cubic feet of soil weighing over 44 tons."

First of all, let's make sure we're talking the right bug. Take a look at this page of Images of leaf cutting ants and see if that is what is going on in your yard.

The most recent information we found on those ants was that they were threatening pine forests as near as East Texas and on east into Georgia and Florida. They apparently choose one particular plant to chow down on, but the reason they take so many leaves is not for food and not for nest lining but for their own version of farming. They go back to the colony, chew the leaves and spit them out and on this base a fungus grows which is the only food of the ants, including the young. When a queen ant flies, she takes a "starter" from this fungus, establishes a new little hole, and plants her garden. Then, the workers come in and begin the fresh tunneling. Because they eat nothing but the fungi, putting out bait is not successful. Because they have such a large and intricate colony of underground tunnels, spraying down the holes in the conical mounds doesn't go far enough to do any real good.

So, you and probably your neighbors have a major problem.  We suggest that you go to the website for the Texas A&M AgriLIFE Extension Office for Aransas County, where you will find contact numbers and ask them if this problem is known elsewhere in the area, and what they recommend you do.  We're afraid that changing the plants in your garden will not be any help, because the ants might just choose another host plant. In the meantime, you could always use the old home remedy of scattering diatomaceous earth (DE) on the mounds and the routes the ants are taking. It won't eliminate them, but it will give you some feeling of revenge, because the DE scratches their external protective covering, drying it out and killing them.