Best Route: Switzerland

Going to Munich first feels more logical to me, but total rail travel time is similar with either plan - about 18-19 hours for three train legs if the Swiss base is around Lauterbrunnen.

That doesn't include extra time for a special scenic train, but one that would fit is the Golden Pass line, westbound Interlaken-Zweisimmen-Montreux, before continuing to Paris.

Another route you could consider is Rome > Florence > Venice > Munich > Switzerland > Paris > Amsterdam. A few thoughts.....

Transportation in Italy is very easy between R/F/V using high speed trains. For example the trip from Rome to Florence is only about 90 minutes.

The trip from Venice to Munich is relatively easy if you use the one direct train that runs each day. It departs Venice at 13:50 and arrives Munich at 20:25 (time 6H:35M). It's a bit of a longer rail trip but the ride through the Brenner Pass is very scenic.

The trip from Munich to Interlaken Ost is not too difficult. Although it means an early departure, I'd probably use a departure from Munich at 06:33, arriving Lauterbrunnen at 12:55 (time 5H:55M, 4 changes). The first leg of that trip is by Bus, but the others are all by rail. That would get you to Lauterbrunnen in time to check-in and have a nice lunch before doing a bit of a walkabout to admire the beautiful scenery.

For the trip from Lauterbrunnen to Paris, I'd recommend travelling via Basel SBB. There's a departure on the current schedules at 10:02 that would get you to Paris Gare de Lyon at 15:38 (time 5H:36M, 2 changes). It's a very easy trip!

There are numerous direct trains from Paris to Amsterdam every day, and the shortest trips are 3H:19M via high speed Thalys trains.

For scenic trains, one of the best (IMO) is the Bernina Express although in this case it doesn't fit well with your Itinerary. You could consider the Golden Pass route from Lucerne to Interlaken, on the way from Munich to Lauterbrunnen - https://www.myswissalps.com/goldenpass/gettingthere .

Good luck with your planning!

If you have a month, may I suggest that the Venice to Zurich/Lucerne/Lauterbrunnen leg is quite long, as is Munich to the same Swiss region. If you broke it down into two segments Venice to Innsbruck (4+ hours) and Innsbruck - Zurich/Lucerne/Lauterbrunnen (4+ hours) you would have the time to enjoy some Austrian scenery too.

My option is admittedly colored by our very long train journey from Munich to Zurich which ended up being substantially longer than listed because of the changes of engines at the German-Swiss border. Maybe just our bad luck.

https://www.bahn.de/en/view/index.shtml

The Swiss scenic trains:

https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/grand-train-tour-switzerland.html

Another possibility to break the journey is to go from Venice to Lugano Switzerland (4+ hours) and then do the day trip on the Gotthard Panorama Express to Lucerne, which includes a Panorama train across the old twisty train line through the pass and a boat down the Lake of Lucerne.

Sounds like a great trip!

Thanks very much for your helpful recommendations. In my mental map of things, Switzerland was looming as this big barrier between Italy, France, and Germany, but my wife really wants to visit it, and now I'm beginning to get unstuck.
From your recommendations, I roughed out several options (the travel times are only approximations at this point):
Option 1:
Leg 1: Venice to Munich (6h 24m)
Leg 2: Munich to Zurich to Bern to Interlaken to Lauterbrunnen (total 8h 3m) (Is the first part of this leg via bus?)
Leg 3: Golden Pass from Interlaken to Zweisimmen to Montreux (3h)
Leg 4: Montreux to Paris (5h 8m)

Option 2:
Leg 1: Venice to Munich through Brenner Pass (6h 35m)
Leg 2: Munich to Lucerne (6h 26m) (Why is 1st leg via bus?)
Leg 3: Golden Pass from Lucerne to Lauterbrunnen (2h 30m)
Leg 4: Lauterbrunnen to Basel (2h40m)
Leg 5: Basel to Paris (3h3m)

Option 3:
Leg 1: Venice to Milan (2h 25m)
Leg 2: Milan to to Tirano (2h 32m)
Leg 3: Bernina Express from Tirano to Chur (4h)
Leg 4: Chur to Munich (5h 6m)
Leg 5: Munich to Paris (5h 41m) ?

Option 4:
Leg 1: Venice to Innsbruck (4h+)
Leg 2: Innsbruck to Lauterbrunnen via Zuruch & Lucerne (4h+).
Leg 3: Which Swiss scenic train? Perhaps from Interlaken?
Leg 4: Lauterbrunnen to Zurich or to Lucerne
Leg 5: Zuruch to Munich (4h 44m) or Lucerne to Munich (6h 51m)
Leg 6: Munich to Paris (5h 41m) ?

Option 5:
Leg 1: Venice to Milan (2h 25m)
Leg 2: Milan to Lugano (1h 8m)
Leg 3: Gotthard Panorama Express from Lugano to Lugano to Flüelen (2h 30m)
Leg 4: Flüelen to Lucerne (3h via boat ---or faster by bus or train?)
Leg 5: Lucerne to St,. Gallen to Munich (6h 56m) or Lucerne to Zurich to Ulm to Munich (6h 51m)
Leg 6: Munich to Paris (5h 41m) ?

What logistical drawbacks do you see in any of these options over the others? Any scenic advantages of one over another?

It also occurred to me that I could reverse my itinerary's starting point and fly into Amsterdam. In that case, my itinerary might be Amsterdam---Paris---Switzerland----Florence & Rome----Venice----Munich. I could fly out of Munich (if there was a good fare) or go to Frankfurt and fly out there (as I have a nephew working in Frankfurt I could visit him, too). (I initially didn't want to start in Amsterdam since this is a spring trip and the weather might not have warmed up yet. At this point in my planning, I'm able to be flexible about when I start the trip and could shift it toward the latter part of April.) Any comments on this option?

Hi,
Venice-Munich is also served by overnight trains. Comfortable Austrian rolling stock. Would you consider them? It would save you a travel day.

Yes, I would consider an overnight train if I should choose an itinerary option that includes a Venice---Munich leg. Thanks for the suggestion.