Flamestrike
Legend
I'll probably go w/PH primarily, with exceptions on a case-by-case basis. There are some options in Xanathar's that I really like.
If you notice class imbalance, it'll likely be down to the number (and frequency and type) of rests you allow.
Warlocks, Fighters and Monks are Short rest dependent. All Casters, Paladins and Barbarians are Long rest dependent. Rogues are 'rest neutral' in that all their abilities are 'at will'.
Accordingly you have a few levers to pull there to move the spotlight around the table, or to mix up class balance without changing a single rule; simply increasing encounter and short rest frequency will favor those short rest classes. Decreasing either will favor the long rest classes.
5E is a heavy resource management game (mechanically). Spell slots, rages, action surges, Ki points, superiority dice, hit dice, hit points, wild shapes, sorcery points, smites, lay on hands, second winds etc are all resources.
Cant stress this enough mate. How you choose to manage this aspect of the game as DM will have profound consequences for how it plays at the table.