I'm
Originally a Welshman so know a lot about mountains! I've also
done a lot of the big climbs on the Tour de France (ventoux, alp d'huez etc).
but never had the pleasure as yet of being assisted by a granny ring and
3 panniers!
You're from Wales. The tallest mountain in Wales is Snowdon at 3,560 feet at the summit. Denver Colorado, which is not in the Rocky mountains but is near the Rockies, is at 5,280 feet. To get over the Rocky mountains, you climb UP from Denver to the passes. The bottom of the valleys between the Rocky mountain passes are 2,000 feet or more higher than the very tip top peak of the tallest mountain in Wales. There is no snow on your mountain all year long. The mountains in Wales are comparable to the Appalachian mountains in the USA near the east coast. They are short mountains. Steep but short. You might climb a couple miles to the top. Not 7 miles of climbing like the Rockies. The mountains in Wales and the Appalachian mountains do not have the same winter conditions, same year round weather extremes, that the taller Rocky and Sierra mountains out west.
You rode in the Alps. Likely in summer. July and August. Not winter and not the iffy months of spring and fall where the weather can change instantly. The Alps are similar to the Rockies. Same height and same weather. There are very few people who plan cycling vacations in the Alps in early May or late September due to the potential bad weather. You would not want to plan a ride in the western US mountains at that time either. You could have wonderful weather. But you could also have a blizzard.