But Shadowheart has one big issue.
She is a Trickster Cleric, which doesn't get heavy armor proficieny, and she has no dexterity to speak of.
So there is no way to give her good AC, unless you get armor proficiency with a feat.
Simple solution would be to give her either 14 dex or heavy armor proficiency. Neither would be extraordinary. For example, wyll is a warlock with proficiency in rapiers, and warlocks can't use rapiers without special training.
since she doesn't have any proficiencies in dex skills, i would just give her heavy armor prof.
trickster cleric already is one of the weaker dnd5e subclasses. heavy armor wouldn't break that at all.
Agreed.