I have been to both glacier NP and Banff last year. If you have the time it is definitely worth it as a scouting trip. Highlights in Banff are Johnston canyon, lake Louise and if crowds don’t bother you, bow falls right in the town of Banff. As for going through castle junction straight to radium, or going through golden, both have great things to see. I have heard that emerald lake near field is beautiful. We like the canyon RV resort in radium and tunnel mountain in Banff. The are pretty rv park like and less rustic, but that works Us with the young kids. Lots of the small parks Canada campgrounds close mid sept, but their website has the closing dates on everything. Pack winter and summer clothes, easy,to have -10 C and a foot of snow and then +30C two days later in Banff in sept.
Waterton is also worth a stop, it is basically the Canadian side of glacier. Red rock canyon was really cool, the rocks in the water were the colour of raw beef.
We crossed at the chief mountain border crossing, but it isn’t opened year round so check ahead.