Last time that I returned to Seattle from Jasper, I took 5A from Kamloops to Princeton, and then 3 to Hope.
5A rejoins 5 at Merritt, so could be an alternative on that climb. It might actually be a bit lower, since it follows the shores of some lakes. But it's curvier, and may have shorter steeper grades. I was surprised that there was some truck traffic, since I figured they'd all take the freeway.
To use the south leg, you first take 97C which is now all divided. No significant climbs or drops, but it curves, especially around the Allison Lake PP. But the first climb west bound out of Princeton on 3 might worry some big RVs.