I very much appreciate you caring about privacy and thinking about it from the get go.
Actually privacy concerns with other watches is what brought me to bangle!
So I would vote against enable by default and for a pop up (if upcomming once and not every time).
That said, knowing your intents and pseudonomisation I am happy to provide data here. I did turn on, re-connected, then off again. Not sure though if that was enough (guess so from your ">Pretty much all of the above")
What I personally would like most is a button: "Send now" (which could be annotated with explanation of what data, how anonymized and how used).
Maximal control, transparency and ease.