As others have stated, there is a relationship between ISO, Shutter and Aperture. The more auto you use, you are letting the camera make choices and often make bad decisions. The fact that you are getting better pictures in auto is likely because you don't understand the relationship, triangle of ISO/Shutter/Aperture. This understanding will vault you from ordinary to extraordinary.

The reason why anyone comes into these forums is to improve on their work. Learn to shoot in all modes and avoid auto for the best possible work, work you will proudly share, print and frame.

There may be few situations but there are some.
I'll give you an example. Some time back I was shooting a small group of birds in bright lighting and I wanted the entire group in focus. I selected aperture priority and f16 and as an experiment I also choose auto ISO.

The camera selected a high shutter speed (which I didn't need)
And it also selected an ISO of 1600 to get me those settings (which I certainly didn't need)

And for anyone that is knowledgeable enough to select the proper shutter speed and lens opening depending on the lighting and subject mater, they also should be able to competently select an appropriate ISO.

Not necessarily. Napa, Sonoma, Anderson Valley, Alexander Valley, and Dry Creek area are all pretty wonderful, and very good for photos. As is Lake County to the northeast. The biggest problem is heavy tourist traffic (more so in Napa and Sonoma) that time of year.

If you are interested in wine country photos, and because you will be spending time in Yosemite, you might consider driving over to the Amador County and Placer County wineries. Some very lovely wineries set in the Sierra foothills. Good wines and much lighter crowds.

The thing to keep in mind is that there is a wealth of great places to see in NorCal, but distances can make trying to squeeze too much in a reality. You don't want to spend the majority of your trip driving and not taking photos. Remember, Google Maps is your friend!