If you want a Rock move that badly, use Stone Edge. It's significantly stronger.
Assault Vest lets Conkeldurr tank special attacks more easily, even some specially-based Psychic and Flying moves. There's literally no need for Focus Sash.
The main use of Mach Punch is to let Conkeldurr pick off frail and/or weakened opponents before they can even hit Conkeldurr. It's pretty much mandatory on it given how slow it is.
Focus Band is a luck-based item in competitive play, and that's frowned down upon.
For a better Dark move, look no further than Knock Off. It removes items from opposing Pokemon; if timed right, it can cripple item-dependent Pokemon. It also gives Conkeldurr a way to significantly damage or KO any Psychics and Ghosts switching in.