I should probably point out that we are the GPs and momma is really GM though we could have the whole fan damily in the vehicle - 6 - a very few times.
I don't particularly care one way or the other on 18" versus 20" wheels (which go with the 8 PAX and sunroof delete no cost option) - 18" tires are probably less expensive to replace.
The kids travel well now with GM in the front seat (station wagon) and, like you said, the infant will be a toddler soon enough. If attention is needed - rarely - we just pull over.
My hunch is in these cases a decision to go without the 8th seat is best.