Granular weed killers have problems with leeching into the soil and lower water systems.

As far as killing all weeds in St Aug and centipede, I believe all the herbicide companies make something with atrazine in it. Here's a (blurry) picture of one of them.

Generally they all have some purple on the label, but read the label first. And read the spray instructions 3x before you use it. I would walk backwards and spray downwind to keep the spray off of me. This stuff works fairly slowly, but it works on grassy weeds and broadleaf but leaves the st Aug and centipede alone. In about 3 weeks after you spray you'll suddenly remember that you sprayed it because the lawn will be weed free.

As for timing, normally I would suggest spraying in mid April, but... We are fully into spring here in the Texas Hill Country. Normal spring for us is in March, but I have some St Augustine awakening. Wild flowers are a full month early. You can spray the atrazine anytime the weeds are active, but the label recommends only once or twice (at most) a year. And normally I would say April and September, but this year it's going to be earlier.

Thank you for that comment. I actually used that last year, except I had very little results. I had a few dead patches too, but I used it per the instructions.. watched the rain and did that you had said and walked backwards. I thought maybe it was the active ingredient that didn’t help but maybe I was wrong.
I hear what your saying that a spray is better to catch on, I’m just worried about getting no results again.

I had found Spectracide granular that used 2-4d. I am in very sandy soil if that matters. I had thought that maybe the spray application just seeped right passed the roots due to the soil type. Does that make sense or does that not matter much?

If you still have the bottle, check to see if any of the product was dispensed. I've had two of those bottles come defective. I have one good bottle that I pour the product into when I want to spray.

Spray herbicides are only for foliar application. They do not work on the roots, so drenching the soil is a waste of product. Eventually the root will die, but that is due to absorption through the leaves.