We did part of your trip in reverse last summer. US Hwy 89 from Browning, MT to St. Mary is a bit narrow and full of curves. Not much sightseeing for the driver on this stretch. From my map, it looks like the Hwy 49 cutoff
down to US2 has even more curves.
We stayed at the St. Mary KOA. It is a large park with spectac views and nice folks.
They held a repair part for our arrival. They also arranged a Red Bus tour for us from the campground to the crest of Going-to-the-Sun. We enjoyed it very much.
The west side of Glacier is beautiful and well worth the time to explore by toad and/or foot.
After a few days at St. Mary, we continued north on US89 which has become almost straight and level. At Hwy17 we turned northwest toward Waterton. Watch for open grazing along this stretch. Hwy 17 becomes AB6 south of Waterton. There is one long uphill grade southbound on AB6 south of Waterton townsite turnoff but it should be quite doable.
Waterton has a pleasant campground in town but the streets are narrow. Directions are sign posted.
From Waterton, we traveled north through Pincher Creek and then stayed in Sparwood, BC at the town-owned campsite before heading north to Kootenay NP.
By the way, if you need a place to stay near Spokane, the campground at Deer Lake, WA is very nice. We stayed there, too!
Be safe and have Fun.