I haven't read "Forgotten Realms" novels, so can't judge how close/different BG3's Shar is compared to the original, but I found her quite convincing as a Goddess of loss, and desire to return to the primordial nothingness. The biggest difference with solace from alcohol is that Shar actively converts and fights against followers of other religions. Shadowheart's little stories about the Dark Justiciars and their methods, the "education" Shar priests receive in their monasteries and the journals off the forceful conversion of Reithwin are quite believably presented, and sometimes brush very close to historical analogues.
As for Shar's "personality", The book they find in Act 1 : "the unclaimed" really painted a consistent picture of her in my mind. I found her very "believable", both her religious organisation as well as her aspect. Contrary to , e.g. Bhaal and his cult, which makes no sense at all for me.
The only really silly thing around Shar, IMO, was the rumour about relic "the Nightsong" in Act 1. No one in the party or NPC's ever thought of a possible connection with "the Nightsinger". Not even Shadowheart.
