Are you 20, 40, 60 or 80?
Do you have much touring experience?
Because it is way different riding a bike with 50 lbs of gear.
It sounds as though you have little experience in the West.
Learning to deal with highly changeable fall weather in the Rockies as a first-timer is iffy.
And you will not be hitting the Rockies in September, but October.
People die in the autumn in the Rockies every year.
They are usually from back East - and inexperienced.
Even experienced people can be caught in a fast-developing fall blizzard.
https://apnews.com/article/a55805fab...617fa3d8576de7
A friend of mine quit the search & rescue squad -
because he was depressed from searches turning into recoveries.
You probably won't die - but you will be plenty miserable.
Not to mention that most services close by late Sept.
And roads in Yellowstone will be long closed.
I live in Wyoming and have taught in Montana.
I've biked, hiked and skied throughout the region.
Our first snow is in early Sept, then late Sept, then early Oct.
The next one, usually, more intense.
There are gorgeous days in between.
But temps can go from 75F to a wind chill of 0F in six hours.
If you absolutely have to wait to start until mid-August -
then - really - you should ride west-to-east.
In fact, it will be ideal all the way across if you start in the West.
Why try to force a square peg into a round hole?