Should it be a system which should be proactively used for mental health assessment or system which involuntarily monitors our activities to make an assessment?

The main hurdle is the scientific validity of such assessment. Even devices from major manufacturers which monitors physical activity with heart rate monitors, accelerometers have failed the scientific scrutiny due to their inaccuracies. Mental health is a volatile research field, the landscape of mental health research changes rapidly the more we understand our brain; one consensus being there's a whole more to understand about our brain.

There is definitely a need gap for bringing state of the art validated research on mental health to the consumers. You may be interested in similar problems[1] posted on needgap.