Yes, driving down in the evening is a great way to extend your trip a bit. You might want to time things so you don't hit the northern Seattle area until about 7pm. It seems like this is the time when things start finally clearing up a bit so you can sail through there without too much trouble.

If you will be getting an early start on the day your return to BC...and to make this work I do mean EARLY...I would really recommend going up the east side of the peninsula and then taking the ferry back over to I-5. It's a gorgeous drive and will give you some variety. (I always like to return a different way so I'm not seeing the same things whenever possible.)

The ferry can be caught at Port Townsend, it takes you over to Whidbey Island, where you would want to then drive north to Anacortes and then east to Mt. Vernon, then back up I-5 from there.

Like I said, this is a gorgeous drive, will take you around the I-5 parking lot of the greater Seattle area, and should be a fun way to end your trip.