I vote for your first option (purchase the 2019 Italy and Switzerland books, and cut out the appropriate sections).
With only 2-4 days in each Italian city, you probably won't use the extra information in the individual city guides. However, if you are traveling with children, or want more details on shopping, or like his museum tours, then know that these things are much more detailed in the city guides compared with the general Italy guide.
Beware of his Best of Italy guide. It has color pictures, but notably less information than his regular Italy guide.
As for Switzerland, his Muerren (Berner Oberland) and Luzern chapters are really good (other than his over-emphasis on Gimmelwald, that is), so you'll definitely get use from them.
EDITED to add: If you want to find out how each book compares to the other ones (such as Rome vs. Pocket Rome vs. Italy), go to the "Is This The Right Book For Me?" section of the listing in his online shop. For instance, here's Rome: https://store.ricksteves.com/shop/p/rome-guidebook