I use an app called accubattery which can be set to alert (make an annoying noise) when the charge reaches a preset level (I use 90%) which could be used, if the charger was plugged into a remote controlled socket, to tell you when to turn the socket off.
I used to do the same thing with a boiled egg cooker to turn it of when it started beeping so can't see why it wouldn't work for a phone charger.
You'd think by now, with phones being able to do virtually anything, even stir tea if your cups big enough (or phone small enough) that it would be possible to get them to stop charging using an app to turn off the phones charging circuit, or even the thing realising it's full so should stop, but no - apparently this is beyond the reach of current technology, despite it being something literally everyone would want, and every other non budget device on the planet managing to do.