All I find is suggestions from a few years ago, so probably outdated or maybe there appeared some better tools in the meantime, there are some apps in the play store but I need something good privacy wise, no training demon, also small battery life impact would be nice
Double-pressing the power button to open the camera should be an option you can enable in Settings
It's an option in my OnePlus too but Motorola's gestures have the upside of working consistently everywhere, every time, while double press power works only when screen is off (on OnePlus at least) also it clashes with emergency menu in OnePlus (5x power button)