Does it rain a lot at that time of year up that way or is it just cold?
I would expect rain on at least some days, but not most days, in Sept, especially near the coast. The BC interior, say around Kelowna or Kamloops for example, is drier, but still gets some rain.
Temperatures at low elevations should be mild. The interior is both a little bit warmer and colder than the coast. If you're camping at high elevation in BC, Manning Park for example, it'll be cold at night, as you might expect at Jasper as well. Could get snow way up there. The weather forecasts for towns seem pretty reliable 2 days in advance but our mountain weather changes rapidly and can have microclimates.
Since you're coming from California, do you get to drive much in the rain? If not, it might be helpful to know that worn, dirty road markers and a lack of reflectors on many BC roads can make it very hard to see your lane on rainy nights here. Something to consider when scheduling your departure/arrival times.
Route comments... The bumpy concrete surface of the I5 between Seattle and Vancouver can be a bit tiring in my wife's Mini, so I can imagine the Elise being genuinely uncomfortable.
I'm not sure about the clearance on the speed bumps at Peace Arch border crossing, haven't tried in my car yet. The lanes are narrow so you might not be able to hit them at much of an angle. The truck crossing further east might be the safer bet. Fill up the tank in Blaine before crossing the border, gas is cheaper there
Hwy 1 from Vancouver towards the interior is a commute, but you could take a more scenic route to bypass it, maybe hwy 7.
Also good to know is that BC has "excessive speed" (40 km/h over the limit) and "stunting" (drifting, etc) laws that result in your car getting towed away. There are often speed traps on the highways approaching towns. Squamish and Merritt come to mind.
There are a bunch of red light and speed cameras being activated at high-collision intersections around the province as well, which should show up on google maps (there's also road signage and a BC gov web page listing the locations).
Have you checked out the Porsche GTS Routes app? It lists some fun drives around here. Sea to Sky hwy to Whistler is a popular one, with some scenic rest stops. It's quieter in the early morning and later night. The viewpoint for the Tantalus mtns is really nice.