We are starting in Paris and ending in Florence. Early Sept. We have a week in between. No car. Train or plane only. We are two seniors. We enjoy finding places away from crowds.Totally enjoyed Santa Margherita Ligure, Beaulieu-sur-Mer on our last trip. Husband enjoys 4-5 star hotels. Any suggestions?

What country do you want to visit -- from Paris to Florence by train you can choose easily between Switzerland, France or Italy. Plenty of nice hotels in all three places

We are open. Looking at Paris -Zurich-Lugano but hoping the forum can suggest towns we might not be aware of. Also Switzerland is very crowded early Sept.

For me it always starts with "what do I like to do", not "where do I go to.." Since I love open spaces, hiking, lakes, vistas, nature more than I do cities & crowds, I would choose a small resort town in the Alps in any of the countries mentioned above.
Since you prefer 4-5 star (I assume that's a US rating system implying luxury and amenities) perhaps start with a list of propreties that meet that criteria and go from there.

Do you plan to move around every day, or stay in one location for most of the week?

If you want ocean, then pick a beach resort along along France or Italy.
If you want mountains, the alps are your playground
If you want big cities, then pick a big city.
etc, etc, etc.

Since that option involves flying, you could also consider flying Paris to Rome or Naples and then heading by train to somewhere smaller/quieter. FWIW, I think anywhere that is easily accessible is very crowded in early September.

Rome and Naples will be very hot in September. If it were me, I'd go to higher elevations in hopes for cooler weather. I'd do Switzerland if it were my trip.

We loved the Austrian Alps in September. There are a number of Almabtrieb (when the decorated livestock are brought down from the higher pastures); and you might even find a grape harvest festival. Taking the gondolas to the high elevations for wandering is lovely, too. The Öztal, for example, has great public transportation.