I know there are several posts on this subject but none are quite what I am looking for and are over a year old. I could use all the help I can get. My wife and I are traveling with another couple in our early 60’s mid-May to early June time frame. Venice and Florence are must sees, but not sure after that. I’m thinking maybe Dolomites and Lucerne, Grindewald and then Zurich. Not sure if it is better to fly into Zurich or Venice first and then fly out the other. Not sure about what trains and passes to purchase up (or down) through Switzerland. There seems to be some concern about how the trains are going to be running.
The first thing to check is flight schedules out of Venice. For some US destinations, departing from Venice means an extraordinarily early short flight within Europe to the transatlantic-gateway airport. The flight from Venice may be so early that use of public transportation is iffy or even impossible. Private transportation to the airport can be very costly, though with four people traveling together it wouldn't be as painful as for a solo traveler or couple.
From the weather standpoint, I'd lean toward starting in Italy and finishing in Switzerland.
I think Venice, Florence, Dolomites, Lucerne, Grindelwald and Zurich would be pushing it for a 2-week trip. How many nights will you be spending in Europe, not counting the night on the eastbound plane?
What do you plan to do in the mountains? If you want to hike, you'll want some advice from experienced travelers (I am not one) who can comment on Mountain-hiking conditions in May.
It may well be that some sort of Swiss rail pass or the Half Fare Travelcard would pay off for you. It's best to have a reasonably precise itinerary before making that decision. There are multiple rail departures per hour from Lucerne to the Zurich airport, at least one per hour not requiring a transfer. Travel time is between 60 and 75 minutes. Therefore, if you don't have an early flight back to the US, you might consider just staying in Lucerne rather than shifting to a hotel in Zurich.
This is the Swiss transportation website; it covers buses and alpine lifts as well as trains (probably also lake boats, but I haven't checked): https://www.sbb.ch/en. Prices default to the fare for those with Half Fare Travelcards.
Both Florence and Venice require changing planes in Europe while Zurich has many direct flights from East Coast US cities, which can be a great plus coming or going, Rome also has many direct flights. You might start by looking at a map and plan a route that doesn't back track by much. But for two weeks I would strongly encourage no more than four locations plus your departure city for the last night before departure. Maybe fly into Rome and take the train to Florence to start the trip (it's 90 minutes) for nights 1-2-3-4. Depart for Venice on day 5 (the fastest train takes about 2 hours) and stay for nights 5-6-7-8. Depart for Bolzano as a gateway to the Dolomites on Day 9 (about 3 hours). It takes abut an hour by bus to Ortisei and a bit longer to go deeper into the Val Gardena. Stay for nights 9-10-11. It is a good six hours on the train from Bolzano or Florence to Zurich, although the trip is through magnificent mountain country. If you're good about that, on day 12 head to Lucerne/Zurich after the Dolomites and spend the rest of the trip among Zurich, Lucerne and maybe the Berner Oberland.
Of all the places to worry about train service, Switzerland is last on my list.