first of all: i'm not really planning on continuing this project, though i found the main reasong of the high battery usage: namely the mcu clock!
found out thanks to a teensy 3.2 running in 48 and then 2mhz with a multimeter attached.
48mhz: over 25mA continous
2mhz: less than 3mA continous
so, in theory if this also counts for the samd21 (the zerowatch mcu), i could get battery life up to a week or more!
anyhow, since i don't got that much time right now, i'll keep it with this, bye!