All I find is suggestions from a few years ago, so probably outdated or maybe there appeared some better tools in the meantime, there are some apps in the play store but I need something good privacy wise, no training demon,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
You can do it with Tasker. I've used it in the past to run tasks when the phone is shaken or flipped. They're under sensor events. Shake has its own event, for something like flip you have to use a generic "Gesture" and show it the move you want.
It's not guaranteed to work with screen off on all phones, you'll have to test on yours. Tasker offers the option of keeping the accelerometer on when screen is locked but some phones override it.