If they are mashing out a low, 6C might be worthwhile to throw out, it goes right over them though this is super risky. On CH you get a combo since its a fatal but outside of a CH or on block its straight back to neutral and they could mash out then. Sometimes if I catch someone guarding with 5A > 5B, I like pausing the 5B and skipping 5BB to do another 5B, its not perfect but it'll catch slower moves (you'll be too far if it was barrier guarded). One thing I've found to be be pretty good at catching mashing is to do something that will put enough distance away from someone (like tossing in 2C which you could delay after 5BB as well) then using 3C or just pausing and back dashing or a slightly delayed forward dash. At the end of a string or somewhere where you except someone to mash out, a delayed TCL could land you a CH as well, same goes for Gustaf or even Valiant/Hornet.