I am a bit confused on what you are trying to do. You had a 4-day route before and now you are talking about a 1-day loop. Can you be clearer what you are trying to plan so we can be more focused in our remarks.
The ride to Tiburon from the Embarcadero is a more interesting ride if instead of "hooking right" at the end of the bike path that starts in Sausalito (and ends in Mill Valley), you continue over Camino Alto (or take the bike path next to the freeway) and then hook right to ride along Paradise Road into Tiburon. This is going the "long" way that most people ride. It is more like 25 miles and has some wonderful views of the bay that you won't see otherwise.
If you want to make this even more scenic, at the cost of some climbing, you can go left just over the GG Bridge and ride up the Marin Headlands on Conzelman Road. At the top, there are some fantastic views of the Bay Area and the GG Bridge. On the (very steep) way down, the views of the end of the Golden Gate (the opening not the bridge) are stunning. You can then rejoin the road to Sausalito and make your way to Tiburon. This will add about 7 miles to your ride, some short, steep climbing and descending sections, and stunning views.
It would be my choice for a 1-day loop around the Bay Area for a fit cyclist.