14 Days Amsterdam, Belgium, Germany, Salzburg

The first thing this plan makes me worry about is luggage. If you intend to see towns or cities during a break from your journey, or before leaving in the evening, make sure either that there is left luggage at the station or that your hotel will be willing to keep it. Also consider how much time you'll lose in the evening collecting luggage from hotels.

I haven't been to Amsterdam, but the sheer number of sites you want to cram into each day looks problematic.

I wouldn't try to do any sightseeing in Brussels or anywhere else in Belgium while breaking your journey between Amsterdam and Paris, you won't have enough time and will be too hurried.

The train journeys that you really need to book in advance for (ASAP) are Amsterdam-Paris (thalys.com), Paris-Heidelberg (bahn.com), and maybe Heidelberg-Rothenburg (bahn.com, trips across regional boundaries within Germany can be surprisingly expensive if not booked in advance). Rothenburg to Munich and Munich to Salzburg and return can be done reasonably cheaply with on-the-day point-to-point tickets, or Bayern Ticket day passes if you want more freedom.

We are concerned that this itinerary might be too much for us and
spending all our time packing/unpacking and on/off trains rather than
enjoying ourselves.

Yes, you should be concerned.

Your flights from/to LAX are a done deal, right? What about that flight from Munich?

1 day in Paris? Really?

You have too many 1-night stays.

Not sure... but it looks like your priorities are A'dam, Salzburg, and maybe Munich (though it's not clear what your plans for Munich are, and sometimes people just go there because Rick Steves or someone else says it's the one place in Germany you need to visit.)

Keukenhof and Zaanse Schans are not in Amsterdam. You need at least one additional day/night in A'dam.

You need to streamline this thing for sure. My suggestion:

Nights 1-4: A'dam

Nights 5-7: Plan something for the Rothenburg/Würzburg/Nuremberg/Bamberg area. Nuremberg is a fine place and an excellent travel base for train travelers doing short outings. And it's a logical spot to break up the long trip to Salzburg. You can day trip to Rothenburg easily enough if you stay in Nuremberg (or in Würzburg.)

Nights 8-12: 2/3 nights in Salzburg, 2/3 nights in Munich

Then back to A'dam as planned.

Visit Paris when you have time for it.

I agree with the previous posters, and have some additional points.
The Keukenhof will be closed during your visit: it's a spring affair.
Skip your planned stop in Brussel. It's be a hassle with your luggage, and you'll only be able to see the Grand'Place and a few things around. Better to arrive earlier in Paris, for which you haven't reserved much time.
It still escapes me why so many people have Munich on their itinerary. First-time visitors might find it an impressive German city, but there is much better around in the same region. Nürnberg would indeed make a perfect base for essentially German places around. (As long as you stay in Bavaria, you can use the Bayern-Ticket: unlimited travel on regional trains at some EUR 30 for the two of you. No need for advance buying.)

Buy Anne Frank tickets today. Too many one night stays. Changing hotels takes more time than you think. Maybe omit Paris if you care so much about Germany. Better, omit Munich and add all that time to Paris. Omit Salzburg if necessary after omitting Munich. Did I say, omit Munich?

I ditto the other posters. Too many locations for the 14 days (13 since on day 14 you fly home). You will get very familiar with the inside of trains. Travel the 1st day to your first destination or you farthest destination (Salzburg) and work your way back to Amsterdam since you have to leave from there. Put all you Amsterdam days at the end to cut down on travel time.

One day in Paris, why not more?
Not sure how easy to find affordable accommodation in Rothenburg. You can see the town in a day.

Why did you pick going to Germany from Amsterdam and Belgium? Why not save Bavaria and Salzburg for another trip and see Brugges, Gent and some of the wonderful villages in Netherlands.

I'm still stuck on Day 1.

What airline and flight number gets you there at 17:30 (5:30 pm)?

Airline Flying from Los Angeles to Amsterdam is Wow Airline- YIKES!! But got great Black Friday deal!

We are thinking of deleting Paris from trip plans since we have been there twice and haven't been to any of other places on itinerary. Also, we are also thinking of taking another suggestion to start at the end of our trip in Munich and work our way back to Amsterdam for end of our trip since we MUST fly into and out of Amsterdam NON CHANGEABLE - this change freed up some time - also we are thinking of replacing the day trip to Brussels with 1 night in Bruges on way back to Amsterdam at end of trip as we were thinking of doing this as a day trip from Amsterdam originally.
So here is how that would change itinerary:
Day 1: (From LAX) Arrive Amsterdam 5:30 pm
flight from Amsterdam to Munich 7:30pm arrive Munich 9:00pm ( we did something like this last year in Bali and it was manageable)
Check in hotel Munich stay 3 nights
Day 2: Sight see Munich (stay the night)
Day 3: Sight see Munich (stay the night)
Day 4: Train to Salzburg - arrive noon(Sightsee Salzburg Old Town, Mirabell Palace, Salzburg Cathedral, Hellbrunn Castle, Salzburg Fortress). (Stay 2 Nights in Salzburg).
Day 5: Sightsee Salzburg (stay the night)
Day 6: Train to Rothenburg ob der tauber (arrive afternoon -sightsee - stay 2 nights)
Day 7: Sightsee Rothenburg ob der tauber stay the night
Day 8: Train to Heidelberg (arrive afternoon -sightsee - stay 1 night)
Day 9: Sight see Heidelberg 8:-12:00pm train to Bruges-Train to Bruges arrive 5:00 (stay 1 night)
Day 10: Sight see Bruges 8:-4:00pm train to Amsterdam (arrive evening -dinner - stay 4 nights)
Day 11: Sightsee Amsterdam (Rijksmuseum, Vondelpark, Albert Cuyp Market, Zaanse Schans, Keukenhof) (Stay Night in Amsterdam)
Day 12: Sightsee Amsterdam (Anne Frank House, "nine little streets", floating flower market, canal cruise)(Stay Night in Amsterdam)
Day 13: Sightsee Amsterdam (any of above sightseeing didnt complete yet)(Stay Night in Amsterdam)
Day 14: Flight to LAX 12:45pm Amsterdam Airport

Does this itinerary see a bit better??
Thanks for all your assistance on this :-)

So Heidelberg and Bruges are like overnight day trips ( 1 night each)

Salzburg, Rothenburg, 2 nights each

Munich 3 nights

Amsterdam 4 nights

That 7/25 revision looks doable. You still should think about the fact that after the train gets into Bruges, you might have an hour on the local bus and walking to your unknown hotel and checking in. I'm not saying it spoils the plan, but be aware that there is still hidden overhead. You prioritized Bruges, and you're entitled to allow 5 hours on the train, plus overhead on both ends of the train trip.

Presumably you understand that bargain Intra-Europe airlines may have substantial charges for baggage and seat assignments and so on. But those extra flights are good way to deal with your cheap overseas, unchangeable flights. You also have to deal with advance purchase pricing strategies by the airlines. The trains rules are more predictable. (Note that advance train tickets are normally unchangeable, non-refundable in exchange for the substantial discounts.)