I was also struggling with drift and negative values. I also receive a traceback error when I invoke tare(). The function works, when I go in and edit the python code (removing the parathesis). I suggested an update to the GitHub.

I found the miniscale.py definitions in the firmware uiflow-micropython project. It has more details on the implementation of object class.
https://github.com/m5stack/uiflow-micropython/blob/master/m5stack/libs/unit/miniscale.py