An adventure. I agree - slow tours lend themselves to flexibility and changes en route.
I'm not a big fan of avoiding the beauty and small towns of the big mountains, but then I recall trying out a hand-cranker a few years ago.... You'll like the desert and coastal US.
From that limited experience, I'd be inclined to avoid the big hills and simply take the Southern Tier route, until I reached the east coast and then take the Atlantic Coast route. You might save enough time avoiding the epic climbs of the Rockies and Appalachians that the additional miles are accommodated in your schedule.
As regards route planning en route, my phone and computer maintain a fair library of pre-loaded GPX files including quite a few of the ACA routes. These can be pulled up an compared with my chosen route. I use Ride with GPS, REVER (a motorcycle-oriented app with good reviews of smooth, twisty back roads) and Google maps to flesh things out.
On the slow tours, I carry a small Surface Go2 computer, so I can keep track of fires (out west), route changes noted on the ACA forum, and play with larger digital map formats. (I also like to look at large paper maps, and mount a small Garmin eTrex to simply keep me on route while I enjoy the scenery.)
As you know - you'll feel a lot better once out on the road, but I recommend you play with enough apps and technology to feel comfortable at reviewing, selecting and combining routes from ACA, ride with gps, crazy guy on a bike, etc. Even if you stick to the ACA routes for the bulk of your trip, you'll welcome the ability to successfully get off route and keep plans flexible.