You could probably leave them in the ground in your zone, but read up on other posts/links about digging and dividing if you want to expand your number of plants.

You could get a stake of some kind and place it a foot away from the base of the plant, firmly into the ground. Then use some pantyhose, tights, T shirt strips or some other flexible tie that won't cut the stalks/stems, to wrap around and keep the plant branches more upright, secured to the stake.

If you cut off the dead flowers where the stems emerged from the plant, it should stimulate more growth and you could get more blooms. Look for new little green growth on either side of the stem you're cutting off and avoid damaging them. Or look for them at the next junction down the stem. They can go on to produce two new flowers, at least. Some dahlias will just fatigue and start to fade away, lose leaves as they shut down for the season. I don't know how they behave in Texas.

Good luck and congratulations on your blooms so far.