Assuming you're not spec'd into Boulderfist and don't have 4p T19, yeah, you'll use FoA in single target. Even with 4p T19 you'll probably still use FoA single target for non-Boulderfist builds - it just won't be a large gain.
For mythic+'s you'll probably want to use Tempest instead of Overcharge, unless you're more worried about the boss encounters instead of overall clear speed (Overcharge beats Tempest by ~3% in 1-target sustained but loses to Tempest by ~5% in 5-target sustained with the default sim profile).