Putting down grass seed will help to fill things in. Probably best done in fall if you are in a warm climate.

thanks callirhoe123. But for now, should I be doing anything other than letting the water/mowing do its thing?

Watch out for more weeds. Weeds LOVE bare ground.

pre-emergent pesticide in that area?

Yes, you just let the St Augustine reclaim it's domination of the yard. You do not need seed. In fact, there is no St Augustine seed, so you have nothing to worry about there. BUT, you are watering too frequently for this time of year. You did not mention where you live, but CST with St Augustine means Texas. But if you live in Brownsville I would have different expectations than if you live in Pampa.

Watering: Water deeply every time you water. Deep means 1 inch all at one time. But you cannot do that twice a week this time of year and get away with it. You should also water infrequently. Infrequently means once per MONTH with temps in the 60s, once every 3 weeks with temps in the 70s, once every 2 weeks with temps in the 80s, and once every week with temps in the 90s. With temps in the 100s, then you can water every 4-5 days.

Mowing: St Augustine should be mowed at the mower's HIGHEST setting.

Fertilizer: You can use Milorganite as often as you care to. Being organic it will not burn the grass. I would suggest the first dose in May, but you've already done one app, so do it again toward the end of May. Then again in September and November.