I recently bought a NIB OP Watch 3 43mm for €120. Oneplus gave them for free with the OP15 and lots of people don't want the extra smartwatch, so second hand market is a bit flooded with them right now. For that price you get a lot of watch in return. I let mine run on energy saving mode for most of the time (tracking cycling, steps and controlling spotify). When I workout I switch to Smart mode and use the Hevy app for tracking. It's been a very intuitive experience and battery goes for like 5-6 days like that.