Searching for NRF52840 power saving possibilities

it is not needed, it goes to sleep automatically once there in nothing to do in event loop

EDIT:
or you can check what bangle poweroff does here https://github.com/espruino/Espruino/blob/master/libs/banglejs/jswrap_bangle.c#L4078
for lowest power state, but not sure this is something you'd want. This wakes up from off state by gpio trigger, not sure if RTC could do it too and whether with RTC turned on it would save something compared to ordinary sleep with advertising off.