RTC goes slow - three min within a day

I've got one more question: is the RTC influenced by turning the bangle off at night?

Yes, it would be. I think after a reboot the RTC gets completely reset and Bangle.js has to reconstruct the time based on what it was at the last RTC 'tick', so it may well lose some time there.

Personally I never bother turning it off - if it's not moving it goes into a low power mode, and assuming your watch face doesn't use a bunch of power they can last for well over a month,