"Our only real plans are leaving Bruges on 18 June and arriving in Venice 10 days later on 28 June."
?? Maybe I don't understand, but it seems like you do indeed have very firm plans, since you responded to my alternative suggestions with this:
"We want to cruise the Rhine for a day, have reservations to stay 2 nights in Rothenburg, see Neuschanstein, and spend the other days and nights in Switzerland."
With no prior experience in Germany, you plan to stick with the same plan you have in your original post, the one based on Rick Steves' most highly-ranked destinations, which I'm sure seems logical to you. Based on what I know about your time frame and Germany, I've advised you differently for the Germany part.
Now I'm struggling to piece together the small pieces of your total plan that you've shared so far...It sounds like you have only 4 nights max in Germany (June 18-21) with the 22nd in Reutte. Then presumably the 23rd to the ??th in one or more unnamed towns in Switzerland. Is that right? I wonder where - and if you are truly panicked, maybe you don't yet have any idea where. And we now learn that you plan to visit Italy - specifically Venice - by June 28. Is that about right? And are you going anywhere else after Venice?
"We want to cruise the Rhine for a day"It does not take a day to have a nice Rhine cruise - only about 1.5 hours for Bingen - St. Goar, the truly scenic part (5 boats per day for this part, only 3 to Koblenz.) Sadly, my Rick Steves GAS book makes a mess with its cruise suggestions (yours probably does too.) Rick in fact tells readers to cruise the Rhine all the way from Bingen to Koblenz - a long 4-hour cruise through parts that you don't really need to cruise through; some of the boats from Bingen don't go all the way to Koblenz anyway (less demand for the less scenic part = fewer boats!) Then when you reach Koblenz, you're nowhere near the train station - you must take the bus there or a long walk across town.
So... a day-long cruise - or even a 4-hour cruise - though it's what you think you want now, probably isn't the best plan. And I'm confident in saying that the other forum members will largely back me up on this 1.5-hour cruise idea.
There's a bunch of other stuff about Germany that you (and possibly Rick) don't have under your belt yet - and why would you? You're a newbie to the country. Rick's top recommendations aren't tailored to your particular circumstances but instead tend to be informed by the demands of his tour business, his relationships with hoteliers, and his own peculiar set of cliches about Germany. Yet you must choose. If your choice is to go with the Rick-picks and do Germany in 4 days by stringing together a Rhine cruise, Rothenburg, and N'stein, so be it.
Your route: No great transport ideas - no idea where you'll be in Switzerland. You'll be zigging to Reutte, zagging back into Switzerland, then zigging back east again to Venice, as it is now.
More reasonable might be to drop Switzerland and after your first 4 nights visit the German Alps instead - Mittenwald/Garmisch (the Karwendel, the Zugspitze) Salzburg and the Berchtesgaden region for example. It would be possible to do nearly your entire trip with the very flexible GERMAN RAIL PASS if you did this. The GRP is valid in places you probably don't know about, including Brussels - Cologne on the ICE trains as well as Munich - Venice on the DB/OEBB Intercity trains. See these routes on the flyer:
The GRP means no concerns about buying advance tickets or scheduling. You can even buy it in Germany.