Hi SugarPlum Fairy-
If I were you, I'd go for time in Luzern and the Berner Oberland (Jungfrau Region). Each person has there own preferences, but -for me - Lugano is well down the list compared to those places. If I were doing it, I'd go from Luzern directly to Italy. You could stop at Lugano, of course.
The William Tell express route, if I recall correctly, uses a boaton Lake Luzern between Flüelen and Luzern, and a train through theGotthard Pass from Flüelen to Lugano. There is another option to consider on thatroute. There are frequent trains on that route, so you can simply takethe boat ride on a convenient schedule, and catch a train at Flüelen whether it isa part of the William Tell or not. The views in the Gotthard Pass arespectacular from any train.
In the high alps May is thetail end of winter, and many services don't start up for the summerseason until about June 15. About June 1, things begin to stir, butcable ways mostly are not running. Even then, staying in Wengen orGrindelwald in clear weather gives amazing views, and the cograil runsall year round.
Luzern is easier to navigate on foot or by tram/bus than is Lugano.
Luzern has a lot of variety, including nearby mountains. lake, and the city itself. The lake boat cruise - especially in the southern part of the lake - is hard to beat on any other lake, in my opinion. You might do some part of the lake cruise and separate that from the William Tell Express.....
Strasbourg to Luzern by train takes 2 3/4 to 3 1/4 hours.
Strasbourg to Interlaken Ost - 3 1/2 to 4 hours, plus another 34 minutes to an hour or so to Grindelwald, depending on connections at IO, for example.
You can play with the timetable here:
You can see if you would like to go first to the Jungfrau Region, spend 2 or 3 nights, then go to Luzern for 2 or 3 nights, and take the boat/train of your choice to Lugano. You could consider doing it in the other order, too.
If you choose a boat ride while in Luzern, you might later use train only when you go to Lugano. 2 1/2 to 2 3/4 hours, including some trains with no change of train.
Just some thoughts. I'm an elderly grandparent myself, and that is what I would do. And, I'd do it in late June.