It was a few years ago that I did the Keys by bicycle. They were doing road-work on US 1. Now it may be better than Card Sound Road. Before, it had wide shoulders in some places, but cat-eye reflectors were embedded into the road surface across the shoulders. The cyclist would have to ride, thump thump, over the reflectors every five seconds, or watch the road carefully to ride between them because they were about three inches apart, or ride out in the traffic lane to avoid thumping and staring at the road surface all the time to ride between the reflectors. In other places US 1 had little or no shoulder at all. The traffic on US 1 was fast and there was a lot of it.
Card Sound Road is pretty much straight with one bridge. It was narrow and had almost no shoulder that I can remember. The traffic was so sparse it was no concern at all. Being flat and straight, the road gives motorists a long view of you before they get to you.
If they widened US 1 and put in broad shoulders good for cycling, that's the way to go because it is quite a bit shorter than the Card Sound Road route into Key Largo.