Also for a more direct answer to your question: I’m pretty sure there’s no tagging scheme that’s been formally approved. As a POI attribute, power_supply=* was originally made for camp sites but my tagging is a pretty straightforward extension. As its own thing, as you noted, there are quite a few options. I will at least single out power=outlet and caution that that one’s intended for micromapping individual outlet locations, so it’s a little different from the others. As far as usage, I see from taginfo that amenity=device_charging_station has 1073 uses vs. 728 for amenity=power_supply. To my knowledge, those are your options to pick from.
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