> Speech to text is a solved problem, AFAIK with a caveat: single speaker. You'd need to train a local AI to identify all these different voices reliably. No easy feat.

The OP asks for “a device that passively listens to your conversations”, so even if single speaker is solved perfectly (I wouldn’t know, but have my suspicions, certainly for a device worn on the wrist, which means it can rotate, be covered with a sweater, etc), that isn’t enough.