I finally got in contact with my electrician. He suggested splitting my 240V 8/3 (40A) wire in a new breaker box to the current Volt charger (16A max) and a dedicated 120V outlet for the Prime charger. I'll also have him look at grounding the other garage outlets to see if it's just a case of connecting the ground wire. One outlet reports ground, so hopefully not much would need to be done.