Thats a chest strap heart rate monitor though. I use it myself (when running or cycling) but I think it would be pretty uncomfortable to wear it while sleeping or throughout the day as a proxy for an activity monitor. 

On-topic: I use the Polar Vantage M watch myself. I like it in terms of tracking some relevant data (mainly runs & sleep) but not overdoing it. I don't know enough about the privacy preservation aspect. I guess if you do not create a Polar Flow account or never sync the wacht over Bluetooth / via the USB cable, then all the data will stay on the watch, so you should be good. And there's an 'airplane mode' and 'factory reset' on the watch. But I assume all this goes for most sportswatches without wifi connection, not just the Polar Vantage M.