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That's an intriguing idea, I had no idea that was a possibility.

Unfortunately it wouldn't work for my particular usage, which was keeping it plugged into an old but expensive smart speaker as a music player via its lightning port. A smart plug would turn off the speaker along with the phone... But I appreciate the suggestion, as complicated as it is!

it would still work if you used some kind of lighting/audio splitter (yes, that's a thing) into your speaker, but yeah, that's definitely a fringe case

A timer is sufficient. No need to be precisely 45% or 55%.

I can't imagine that a timer wouldn't quickly drift and either drain it to zero or charge it fully

This