Thanks for your comments. I don't doubt a bit that the steeps in the Appalachians and Ozarks can be worse that the climbs in the Rockies. I've seen 18% grades in Ohio (that was the only time since I first started riding over 35 years ago that I had to stop half way up a hill to look for the lung I coughed up), but my issue isn't steepness but altitude. I'll be 70 when I make my transcontinental ride in 2023. I've kept a meticulous log of all my riding for the past 15 years, and I see evidence in the stats of the very slow but inexorable decline in physical capability that comes with age. I still ski (it's been 3 years due to the pandemic) but riding up a lift to 11,000' or 12,000' and skiing down is quite different from chugging up to that altitude on a loaded bike. I may well be able to do it without a problem, but I won't know for sure without actually taking it on. I'd rather not take the chance only to discover I don't have the lungs for it anymore, and have to reroute at that point or cheat a ride over the top in the bed of a pickup. I'm looking for ways to improve my chances of successfully completing my ride, and avoiding extreme elevation is one of my considerations.
I appreciate your suggestion of the Eastern Express. I've heard of it but knew nothing about it until I started investigating after your posts. I've been concentrating on routes available from ACA, and it's not in their inventory yet, though I read it will be soon. I will be looking into it.