Hmm, I have Bluetooth activated all day long. At night the phone switches to Airplane Mode (automated task), which switches Bluetooth off as well.
I actually don't have battery draining problems, besides I'm not a heavy user. - Battery lasts round about 2 days.
Just thought Batterylife could be even better if phone would fall into Deep Sleep, at least at night (in Airplane Mode).
First i killed that transmiiter-app (for the rodmi 3s), i've allowed it to autostart.
Second i switched all (i hope) apps, i changed, back to "Stromsparen (empfohlen)" in german, that "optimized" option. I'll try tomorrow whether that roidmi app is realy the showstopper.