You can rock up at any time after the previous ferry has gone, park up in the queue that is bound to be there, and enjoy a lovely evening.
In the morning, the first member of staff to 'man'??? (ours was a very nice lady) the entrance to the roped off area to do the primary check of tickets ( doesn't apply to you) will go down the lane giving a cheery 'bang' on the side of the units to wake you up. Plenty of time for a cuppa before loading.
BTW Plymouth City Centre is very walkable from the ferry terminal on the evening before with it's many bars restaurants etc.
You lucky so and so. Did it on this weekend, last year.
We're also on Friday's 8.00am ferry and planning to arrive at the ferry terminal at around 9pm tomorrow. The Pont Aven is leaving at 8.45pm tomorrow so not advisable to get there before that leaves although there is a graveled area on the right hand side where you can park for a short while. But you'll probably be surrounded by lorries and not sure whether it's sensible to leave the van unattended. But after the Pont Aven has departed you can move into the queue.