Basel to Venice with 4 nights to plan! 25th wedding anniversary

The Bernina Express is on the route to Venice so go with that. Bern and Zermatt are outliers and will increase the amount of travel time that’s precious. You need five nights to explore the Lauterbrunnen Valley, especially since the weather could cause delays in sight seeing.
There are direct trains from Basel to Chur. However, you will need to connect between Tirano and Venice’s S. Lucia station at Milan’s central station.
A good day trip from Milan is Lake Como. There’s a direct train from Milan’s central station to the Varenna-Esino station. From Varenna you can hop on a short ferry to Bellagio. Make sure you explore both Bellagio and Varenna.

So you have 5 travel days ( 4 nights) to get to Venice? And that leaves you a few days to spend in Venice before the cruise? Do you know where you actually board the ship and how you are going to get there from Venice? Only the smallest ships can use the docks at Venice, so you likely will have to travel somewhere else to meet the ship. Do Viking provide transport for you snd your luggage?

And speaking of luggage, how much will you have for these 2 cruises? Do you need an overnight somewhere with laundry facilities? Will you have big suitcases that might be hard to get on and off trains, so you need to minimize transfers between trains on the way?

It looks like your Viking cruise ship may be docked at Chioggia. Is that correct?

Keep in mind that at the end of October there is a much greater chance of cloudy or rainy weather in the Alps, and the days are shorter. The Berner Oberland villages may be starting to close down for a break between summer and winter seasons, with limited lodging and dining options. I noticed that the place we will be staying in mid-September closes their kitchen in October, although they still offer rooms. And the same may be true of the village of Pontresina on the Bernina Express route. I note that a favorite hotel there, Chesa Mulin, closes for a break on October 20. This may be an indicator of a general trend.

Moreover, the Bernina Express is a considerable detour on your way to Venice. Take a look at this rail map of Switzerland, with the scenic routes highlighted, and you will see that the Bernina Express route (#1 on the map, in red) is far to the east of Basel. After Tirano, you have to travel back west at least as far as Lake Como (if you are heading to Varenna) or to Lugano. In either case the goal is Milan, where you catch the train heading to Venice. There are no train tracks from Tirano going south or east to Venice—-it is too mountainous.

https://myswisstravelpass.com/Map2024_swissasap.pdf

The Bernina route is a fine way to travel to Venice in summer, with overnight stops in a mountain village in the Engadine, and another in Varenna on Lake Como. We did that on one of our trips to Venice, flying into Zurich so we could take this route that gets so much love here. We spend 2 nights in the Alps and one in Varenna before traveling to Venice on day 4. This was in September, and the weather was 50-50. The first two days, traveling to Samedan and hiking there, were fine and sunny. The next two—-riding the Bernina Express and beyond to Varenna, then taking the train from there to Venice, were a mix of clouds, fog, and rain the whole time. The landscape over the Bernina Pass was all brown and grey, and when we got to Varenna we could barely see the lake from our lakefront room. The sun finally came out late the following day, just in time for our arrival in Venice.

But the Bernina Express is not the only scenic route from Switzerland to Milan. Look at #5 on the map, in blue. this is the Gotthard Panorama Express route, which starts in Luzern and ends in Lugano, a short distance from Milan. This train does not run in late October, but you can take the Treno Gottardo, which runs on the same tracks, ending at Locarno instead of Lugano. This regional train departs Luzern daily at 9:18, 11:18, etc.

https://www.myswitzerland.com/en-us/experiences/treno-gottardo/

https://www.sbb.ch/en/leisure-holidays/topics/treno-gottardo.html

So—-you could spend a night in Bern (or Lauterbrunnen) and 1 in Luzern, or 2 in Luzern, then head south and spend the next night either in Locarno (a lakeside Swiss city) or in Milan (leave the Treno Gottardo at Bellinzona and transfer to a train going to Milan if this is your choice). Day 4 you take the train from Milano Centrale to Venice.

I think my preference would be to head straight to Lauterbrunnen and spend 1 night there. Then take the scenic train to Luzern (about 2.5 hours). I find Lauterbrunnen to be lovely even in clouds and mist, and if Trummelbach Falls is open that is always a good rainy-day activity. This plan will get you at least a taste of the Alps without the potential complications of a mountain village at higher altitude.

If I have mis-counted and you have 4 nights before arrival in Venice (which your title seems to indicate), you could stop overnight at Desenzano on Lago di Garda and explore nearby Sirmione and its Roman ruins at the Grottoes of Catullus.