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I wouldn't've gotten it if it wasn't for a gamestop bundle, but now I like using it to store my spare joycons.
I bought this usb power switch to easily turn charging off when I'm just storing the controllers so the batteries won't get worn out.
Fantastic job. On your issue of single cord, I'm not sure what power your running out of it but I got one of these for my pi so I didn't have to keep plugging and unplugging. Real lifesaver on the old back.
You can just buy one off of Amazon:
Adafruit USB Cable with Switch [ADA1620] [link]
There's probably a better way tho
What about a usb cable with an on/off switch? [link]
I don't see any reason you couldn't tear that switch apart and replace it with a relay if you really want.