When it comes to Shadowheart in BG3, the best feat for her is going to be either Tough or one of the Magic Initiates. The former is quite obvious since she is primarily a healer, so boosting her survival abilities is a nice touch. However, if you want her to be more of a well-rounded spellcaster and warrior hybrid, you can teach her more spells from Warlock or Wizard without multi-classing her.