Red 2019 Ioniq 38 Premium EV
The only "dumb"-ish solution I can think of, would be adding some smarts to your EVSE. I've sat in my 38er charging to 100%; I thought a final top-up 80->100% would take 45 mins; it actually took more like 100 minutes, as the final few% were really slow, with the kW going down to about 2Kw at the end. I can only assume the BMS was doing some cell balancing so some cells weren't taking much at all, or maybe cell balancing is a slow, low-current process anyway. Viridian ECUs used in some EVSEs (e.g. I have retrofitted one to a dumb Rolec) support infinitely-variable charging rates, from 6 to 32A, controlled by an external resistor, or applied voltage. So adding say an Arduino + a digipot, and a current-transformer round the mains wire to the car, would be able to measure actual charging current, and so detect when it's doing the final slow-down. It would have to be adjusted to the particular EV of course, but it could be programmed to stop the charge once charging current dropped below 5Kw for e.g.