“Especially important is the warning to avoid conversations with the DM. We may ask what is relevant but anything beyond that is dangerous. He is a liar.” -Merrin, Cleric of Tymora
All classes have their strengths and weaknesses but some are known to fall behind the mathematical curve. You could change class features but I find it’s easiest to make balance changes with magic items. They’re easier to justify within the lore, easier to change or remove and can be tailor-made for each player. When our Phoenix Sorcerer almost burned herself to death, I felt it made sense to give her fire resistance. She’d later find a family heirloom, made of red dragon scales, that did just that!