I just did a phenomenal ride down in Waterton to the US border (closed since COVID):
https://www.mapmyride.com/routes/view/5235229288
3 big climbs and 3 big descents (hardly any flats/rolling sections), but the scenery was remarkable. Pavement is in perfect condition. I encountered 12 cars the whole time and just one other group of 4 cyclists on e-bikes.
My big mistakes were:
1. starting too close to the first climb so I didn't get any warmup (I did it the day I drove down and was antsy to get riding) - it would be perfect to start from the townsite as drawn on the link (there is a .
2. eating one last cookie before starting, which really multiplied the effect of #1
Even if that border crossing opens up again I think it could be an enjoyable ride outside of peak times (speed limit is 60kph for most of it). Could even be a nice 125km loop if you do both border crossings and go through Cardston...would be the same amount of climbing as doing the short out and back!