How much less energy would it take to make (electric) vehicles out of carbon?

There now exist all carbon PM-free motors, all carbon and copper free cables, and there are organic and inorganic carbon-based substitutes for metal and plastic.

Are bioplastics less energy intensive in the automotive industry too?

Anyone can make a "study". That does not fix the problem with electric.

Just talk to someone who uses that for over a year in real life, not just driving on weekend.

Then talk to someone who understands what is really going on and how much CO2 is spend making the whole car.

At least if you still want to buy one, there are some great options for Mercs, BMW and BYD and many other Chinese companies, not all. Toyota and Honda also have good ones.