213 is the only string that has enough hit stun to cancel into MS Cartwheel, so if you're going for a straight punish string go for that. She has a mean case of t-rex arms and just doesn't hit much with that, but 9f is her fastest full punish.
That being said, if you have a 9f+ window, it's normally just as easy/more reliable to use the overhead to punish; you lose 3f of potential catch, but it won't whiff, more chip/meter, the OH to low catches people sometimes even when they know it's coming, and then another MS mix up in there as well.
Opening people up is something I normally don't have much/any trouble on because of how strong the 50/50 is. Sonya's openings normally come from the 50/50's and not more patient pressure/block/footsies back and forth a lot of other characters have.