Physical controls are the key
Make sure you choose one with physical controls that work the way you want as soon as you connect it. That's actually what the smart plug does – when you flip it a smart plug on and off (or automate it to turn it on and off), you actually only cut and restore the power to what's connected to it (in this case, a heating). That is why you want physical controls that are not reset when the device runs out of power.
Another tip: add a temperature sensor to your setup and then automate the heating to automatically turn off when the room becomes warm enough for your taste, then back on when it gets cold again. You have many options ̵