Cool project! I was thinking about how a person could make their own PHEV conversion instead of just a regular EV conversion so you wouldn't have to buy so many expensive lithium batteries. My thought for efficiency is a parallel hybrid would be more efficient than a series hybrid because of all the conversion losses basically it would do something like what you have going and then add an electric motor to do the round town driving and then on the highway use the gas motor to maintain the battery charge and keep the car going at a steady state that way you have great acceleration thanks to the electric motor but you have range thanks to the gas motor helping move the car along with quick refills of the gas tank.
