I rented a car while traveling in hakone area ; 8 of us inclusive of 1 toddler and it was ok using serena (inclusive 1 baby seat).

I guess can apply to 6+2 or 8 adults as well.

We spend 4 days in tokyo and 3 days in yugawara and the driving experience was the highlight of our stay. 3 days at Hakone area driving up to lake kawaguchiko and small tow of Atami. You can never beat the convenience of traveling by car. Imagine 8 of us including the children needing to waze through the traffic of people getting into the train.
Driving give you that convenience of traveling from point to point and you can cover a lot more in a day. Never mind the toll/ petrol and the parking charges. But i guess you have to be really confident to drive in a foreign land.
The tip is to ask your landlord ( mine was through airbnd) to rent the car on your behalf and you can get your car cheaply (I used 100 yen car rental). By the time you are at the car rental place you will be treated like a big boss. It was nothing like in other countries.
I was actually requesting to rent a car during my stay in Tokyo but my other landlord was against it. But when i dis travel around Tokyo, the traffic is not more congested as in KL and it is doable to drive there as well.
As for the luggage, i just put them inside the room during my travel.
Hope that helps.

I baby seat is one extra seat. You can't book an 8 seater mpv with 8 people with 2 baby seat inclusive. You need aomebody in Japan to help you with this. I has th same experience with ToCoo as well.
Other alternative is book for 8 people but rent the car seats there itself but there is a risk involve as well.