Itinerary help needed

Are going all the way to Fuessen solely to see Neuschwanstein, then immediately taking a night train to Venice? It is a long way to go for a 1 hour tour of a 19th century romantic's idea of a castle built with re-enforced concrete. Hard for me to say skip it, it is your vacation. Its just that is an OK thing to do if you were staying in Munich and were running out of things to do.

Alternative is to go from St Moritz/Pontresina to Milan using the Bernina Express to Tirano then the regional train to Milan that goes along the east shore of Lake Como. In Milan, transfer to a high speed train to Venice.

The night train to Venice may not be as comfortable as you imagine. Stops in the middle of the night, stretches of jerking and shaking. I've done the older version by City Night Line that went via Innsbruck and Verona. Glad we did it, but it was a groggy morning in Venice as it was a restless sleep.

Another thing, rather than going all the way to Zermatt to spend a night for the Glacier Express, stay in Lauterbrunnen and take an early morning train to Brig and pick it up there.

Just random thoughts. Hint, I never post my itineraries on the Forum, because I know everyone will tell me I'm doing it all wrong.

I've been to all these places, including the night train, and I totally agree with Sam.
For example, you could spend a whole day just getting to Zermatt, just to find out that the Matterhorn is covered with clouds (which it seems to be most of the time). Instead, stay longer in the Lauterbrunnen/Murren/Grindelwald areas, where there are even things to do on rainy days (Trummelbach Falls, for example). You could spend 10 days in the area and take a fantastic trail every day. I recommend these 3 for limited time:
Mannlichen to Kleine Scheidegg (best) 2 hrs
Grutschalp to Murren 2 hrs
First to Bachalpsee and return (45 min each way)
Google image 'Bachalpsee' or 'Mannlichen' to see why. Frankly I'd rather take these trails than be on top of the Schilthorn (and cheaper too!).

For other trails, click on my name to see previous responses on the trails.

Would we really be able to get a good feel for the alps if we were to
stay based in Lauterbrunnen area 7/18 - 7/21, visiting Gimmelwald,
Murren, Schilthorn, Jungfraujoch, etc. without seeing the Zermatt to
St. Moritz area?

Yes.

I agree with Linda L. Stay the whole time in the Lauterbrunnen Valley!

train to Switzerland, train to Fussen, Germany for 1 day to tour
castles

Switzerland has great castles, too. These three date back to about 1200:
By Lake Thun, which is a short train ride from Lauterbrunnen, are the castles Schloss Thun and Schloss Oberhofen:
http://www.schlossthun.ch/en/
https://www.schlossoberhofen.ch/en/home
Further from Lauterbrunnen, on Lake Geneva, is the famous Chillon Castle (Château de Chillon):
https://www.chillon.ch/en/

@Sam Thanks for the great information you provided, I'm also planning a trip to Switzerland and I'm still trying to figure out what other countries I'd like to travel to and how to get there.

@shoe Do you have a link where I can find more information on the train? I'd prefer to take the scenic route even if it takes a little longer.

@LindaL Wow I definitely need to stop by some of these castles!

@Dani Hello, it seems like you're somewhat in the beginning stages of planning your trip as am I. I was struggling with how to create an effective itinerary as I'd like to visit Switzerland, Austria, Italy, etc. I just spoke to a friend who's been using a great travel website to figure out her trips. The site gives you many different options to plan a great trip from being able to pick how many days you'd like to to travel to including certain activities within your trip. Although we might not travel the same way, I feel like this site might help you when developing your itinerary. Best of luck and have a great trip. https://www.stridetravel.com/switzerland-tours.html