Add a disconnect not a junction box. Almost same price but it’s way better. Run number 6 and upgrade your breaker to 50 amp later when you want to run the charger at 40amps. Liquid tight is good, try not to use 90s they suck to pull through.

Ok. I’m a apprentice electrician, and I’m doing a ev class right now. My info might be slightly off but I’m fairly confident.

As far as running 6’s then 8’s yes. It works. I wouldn’t use a junction box. I would use a disconnect. That’s code where I am.

Now. The big thing here is. Your car charger that you’re installing Is 40 amp, you can’t use a 40amp breaker. Your running continuous load. Your need 125% for overhead. So, If you have a 40amp charger you need a 50 amp breaker.