As far as I can tell, you're not contractually bound to ATP if you instruct for them. The guy behind the Youtube channel "Prop & Gear" left them shortly after finishing standardization. The CFI I'm working with right now left ATP after a bit as well. He said he would have probably stayed longer if he'd been sent to Florida, Arizona, or SoCal.

You can choose your "base" but, depending on company needs, you may spend some time at another location "temporarily."