I also think a completely new top-level amenity tag is not needed here. At least not for my initial problem, which is a locker where you can (but don’t have to) charge batteries or your phone or whatever. I personally don’t have an e-bike but would still be able to use the lockers as normal lockers and put my helmet in it.

I think a charging station where you can (but maybe don’t have to) lock your device is a different story. But we already have amenity=device_charging_station + lockable=* according to the wiki.

Regarding the question whether or not to create a separate feature for my lockers: IMHO it depends. When mapping a but stop shelter as way, I don’t add the bench within the shelter way as node, but I tag the way with bench=yes. So it’s not uncommon in OSM to add multiple objects as one combined feature, because the separate objects are spatially very close to each other and, furthermore, belong together.