Shar is very very well known and so are her symbols. She has kinda done things warranting that, and part of it becoming an underground doomsday cult as you put it is that her and her worship have been outlawed in faerun and many many people know the symbols on sight and know to avoid them consequently.
I still think shadowheart would be much better if she had a plausible lie and her symbols became just a little more generic. Could claim to worship the dead god Mask who is close to shar. Said lie can be used to gauge our character's reactions, seems in line with trickery domain, and ultimately would seem plausible to those who do not know she is a sharran (even with an added religion check telling us Mask is dead and while not impossible is somewhat suspicious).
Essentially, I want her to be a better liar.
^This. And a big part of why she's been banned in many parts of the Realms is because of her central role in the complete destruction of Sembia in a horribly ruinous war (again strongly recommending The Twilight War trilogy for anyone interested in this lore).
