It doesn't quite work that way. Only DC-charging uses external chargers, but almost nobody has those at home since they're like $60k and require 3-phase power. A/C-charging, which is what most people have at home, uses a charger that's built inside each car itself, and you then just plug it into some sort of outlet at home.
That outlet could be a normal 110V 15A outlet (no way to special tax), but more realistically it's something like a NEMA 14-50 240V outlet. Except there is nothing EV-specific about a 14-50. I have a quite few of those on my property and I also use them to run my Christmas Lights, my RV, my oven, and some 240V power tools.
The J1772 EV specific A/C units that people see in some homes and public places and that they love to call an "EV-charger" is not actually a charger - it's a glorified 240V extension cord with a GFCI box and a fancy plug that fits some EV's natively (not Tesla). I actually still have an old J1772 box lying around from our Nissan Leaf days - but guess what, the other end of that box also plugs into a 14-50. If there is any hint of a tax on J1772 outlets at home, people will just abandon them and use 14-50 outlets directly instead. There would be a sudden uptick of electricians installing "RV Outlets" and "Welder Outlets".
Anyway, point is, at home there isn't anything EV-specific that you can build metering into.
I'm personally fine with the WA state flat EV road tax as it is (equivalent to 12'500 miles/year).