I have ridden nearly all of what you propose.
I believe that you are moderately early in the Rockies -
And you may be slightly too late in the Cascades.
I've ridden across the Southwest and up the Rockies to Alaska a half dozen times -
https://www.crazyguyonabike.com/doc/...c_id=1168&v=sM
I've had to hack my way across avalanche chutes in late June -
Probably a stupid thing to do solo, since there was a raging river below.
And you are talking about the GDMBR - not pavement.
Most of the higher elevation USFS roads in Colorado and Wyoming are snowed in until June.
And even in June, you will have big mud and snowpack.
There are two issues in the fall in the Cascades - fires and early snow.
Since this is expected to be a record El Nino year, fire may not be too bad.
But then, wet years tend to have earlier major snowstorms.
These can occur any time after Labor Day in the Northwest -
And certainly by October in the High Sierras.
April really is lovely in many parts of California, esp. in a wet year.
And by October, the falls in Yosemite are dry - or just a trickle.
I would suggest dialing your entire circle clockwise 2 hours.
Start and end in California - probably San Francisco.
Ride down the coast a few days - then ride into the Coast Ranges at their loveliest.
From there cut across the Central Valley and into the Sierra foothills, awash in green.
Then to Yosemite when it is mind-boggling and the falls roar.
From there you can work your way down the western Sierras and cross over into Nevada.
You can hit the southern Utah parks and Grand Canyon early in the season -
And hit the Rockies in mid June. It still will be tricky, but not impossible.
Frankly, I think you will have plenty of time to finish the loop by late Sept -
Then have time to do something like Route 66 in October to Santa Fe.
But an April state for your current route would be too early.
Best - J