> Your phone probably already does something like this.

It most certainly does not. Most devices track battery health % (last full capacity divided by design capacity) and the gauge just presents state of charge (current capacity/lastfull)

The better phone charge threshold systems measure usage and keep the phone in the 30-80% soc range as often as possible.

Voltage drops faster on old cells as they age so you need a coulomb counter. Only extremely shit designs guess soc based on voltage alone.