Hello Higuexoarr81
The best time to visit Switzerland is between mid of August and mid of October. In autumn, the air is less humid, thunderstorms are less frequent, and the view is better, which makes for great pictures. The so-called “picture postcard weather”, as we locals do name it, only exists in autumn.
When it comes to the Indian summer, the best time is in the first two weeks of October when the weather is steady. As you rightly say, it all depends on the overall weather situation and there is no general saying. In Switzerland, the weather is scarcely predictable for more than two to three days in advance. And, it also depends on the summer, whether it was hot and dry.
From the 20th of October, you will encounter another obstacle: the cable cars, gondolas, and funiculars will be undergoing maintenance, rendering Switzerland less appealing to tourists due to the limited accessibility of mountain peaks (with some exceptions with all-year services like Mt. Rigi), for which Switzerland is so famous.
Please let us know if you have got more questions, or when you are done with the itinerary planning, and it comes to ticket questions. My personal advice: complete your itinerary planning first (bases, sites you are interested in, etc.), and as a last step, choose the tickets.
Kind regards,
Roland