Spokane to Calgary with heavy load - best way?

You cross Kootenay Pass only if you go straight north through Washington. US95 goes through Idaho, and follows rivers with few hills. Crowsnest Pass on BC/AB 3 is gentle, barely noticeable. And traffic isn't heavy, except maybe on weekends when Calgary people are heading to resorts in BC or around Kalispell.

Hwy 3 between Creston and Cranbrook is also easy. Though coming the USA there's little reason to choose the Creston crossing (except for the scenic drive up the lake to Nelson).