Not exactly trying to drop a new bomb in here, but what about adding the 49-mile drive in SF? https://en.wikipedia.org/wiki/49-Mile_Scenic_Drive
This is exactly the type of thing I absolutely do not want this system to scope creep into including. Random insignificant local nonsense that happens to have some signs, as opposed to the major multi-state spanning stuff like Great River Road that was the impetus for the system's creation.
I think San Franciscans might have some quibbles with your assertion that anything in The City is insignificant.
Insignificant might have been a somewhat controversial word choice.
I assumed this system to be "National" tourist routes which would be definition exclude state or local scenic/routes. My home state of Ohio has quite a few "scenic byways" that do not cross state lines. They don't belong is this system either. Admittedly, most are part of the existing state route system, but any portions that are not should be omitted.
There could be additional national routes worth including, but if the base criteria are that the route must be signed and exist in multiple states, that should limit the system to a reasonable scope.