My guess is you do get a guaranteed 6 hours, you just don’t get to choose when within these 6 hours. I.e. they’ll choose the cheapest half-hour blocks to charge (upto what you need) turning the car charging on and off during that period. These HH blocks will also happen to be the greenest times to charge (lowest demand or highest surplus).

Relatively simple to do.
Poll the API to get current charge, target charge, calculate how many kWh needed, add on 5%, convert into how many HH blocks are needed, work out the cheapest HH blocks, use the Tesla API to wake-up your car and turn car charging on and off. Voila. A raspberry pi could do it