FWIW, Darren Alf, also known as as the Bike Touring Pro, just did this ride and route, or at least did it this past May I believe, it’s on his current YT series about 600 miles around Montana. He was on the frontage road most of the way, except for that short section on I90 which didn’t seem to be an issue for him. I seem to recall he said it was a noisy route due to I90 traffic nearby.