"Curious if this itenerary seems too ambitious." No, it's feasible in 12 days, but as others have mentioned, the brief amount of time in Salzburg, Füssen, Baden-Baden, and Bacharach puts tight limits on what you can see/do in - and around - those places. For example... On Day 10, you'll have half a day in Bacharach, which is more than enough time right there. But on Day 11, assuming you want to explore the Rhine/Mosel region more fully, is one day really enough??? Not the way I see it. A day trip to Salzburg is similarly problematic... maybe enough for seeing a few things in town, but not nearly enough time for the really nice places nearby. I could easily see 3 or more nights in each of the places you have picked.
"Any other ideas, omissions, substitutions, or recommendations would be wonderful."
Unfortunately, I haven't heard enough yet about what you hope to see and do. On the surface, your town choices seem to be the same places that Rick Steves emphasizes a tad too heavily in his materials. Or... maybe you have your own reasons for choosing these places... Or... maybe you haven't yet considered your own particular interests, hobbies, or favorite outdoor activities in mind?? Hiking? Cars? Cathedrals? Castles? Roman history in northern Europe? Wine-tasting? Cultural festivals?
I am not a fan of Baden-Baden in particular, but the state of Baden-Württemberg, which you must pass through to get to Baden-Baden, is loaded with interesting places - including dozens and dozens of palaces, castles and monasteries:
https://i.pinimg.com/originals/70/6e/a7/706ea72ac08ab30eebf6a99b900946d1.png
The Black Forest and its many handsome old villages are also in Baden-Württemberg. The Mercedes and Porsche Musems are there too. For all I know, you are a LEGO fan and you'll want to visit Legoland, which is on the route to B-W, or a fan of flowers who would enjoy the Chrysanthemum Festival in Lahr, B-W.
Maybe you're a party guy who would enjoy the huge Sausage Market Festival in Bad Dürkheim, or the Technik Museum in Speyer, as you make your way north to Bacharach and Frankfurt.
So while I get dropping Baden-Baden, it's still possible that you might want to hit Baden-Württemberg for reasons of your own. Maybe more time in the other places, or time in different places, would work well for you. But I can't really say one way or the other without more input from you about the dates of your trip, your travel goals/interests, and other details.