Dungeons & Dragons 5.1E — Monk

[Catullus64]

A tangential point in another thread reminded me to ask whether you think Stillness of Mind is fine as-is. I've always thought it was a bit clunky, as it requires an action to end a frightened or charmed effect... but Frightened, and even more so Charmed, are two of the conditions most likely to control what you do with your action in the first place. Either keeping the action cost but adding in a proviso that makes sure you can always use it, or ditching the action cost and giving it limited use (either flat # of uses or Ki cost).