exempt only affects withholding not your tax liability. you are correct. any unused EV credit not used in the year the vehicle is purchased is lost. by the way, not all EV's qualify for the maximum credit of $7,500. Credit is based on size of vehicle and battery capacity and in addition the credit is phased out as more than 200,000 qualifying vehicles made by the manufacturer are sold in the US.
https://www.energy.gov/eere/electricvehicles/electric-vehicles-tax-credits-and-other-incentives
https://www.irs.gov/businesses/plug-in-electric-vehicle-credit-irc-30-and-irc-30d
