I would not spend much time worrying about it. Just go with the flow.

​​​​​​Especially given how expensive *A tag flights may be ex-STL, why not pick any carrier out of STL, like WN, which has a major presence there, to get to where you need to be. You can arrive the day/night before, or in the morning if you have an overnight flight just to be on the safe side.

In summary, don't stress over it!