Seeking Advice: 10-Day Germany Trip Itinerary

I don't think there is much hiking in the Schrecksee in late March, unless you have snow shoes. Its still winter in the Alps.

Seems like your plate is pretty full. How are you traveling? What are your interests? Düsseldorf?

"...it might not be possible to see everything..."

With that many destinations and cities as large as Munich and Berlin, your optimistic plans need more than just a light trim. For Berlin, most of us here are going to suggest at least 4 nights, I think, nearly half your available time. There would hardly be time to travel to all those other destinations after that, much less see them. Weather does matter... you might want to avoid the Alps altogether. If hiking matters, well, Germany has scenic hiking all over the place, so that might be possible at lower-elevation locations if the weather gods are generous to you.

You can help us narrow things into a more doable plan by providing some focus... the reasons you want to visit Germany, how you prefer to spend your time when traveling, etc.

Well, you have 7 places listed to visit during a 10 day visit. Does the 10 days include your arrival and departure days?
You need to think about travel time between those places. Munich is far away from Berlin and Cologne and Dusseldorf are too.
Perhaps whittle your list down a bit and plan a trip for next year to see the other sites?
You haven't said what your interests are to have chosen each of these places as they are all quite different from each other. Maybe visit the places that are located in the top half of Germany this year and then the bottom half next year?
You also might want to take into account that the weather in Germany in March can be extremely variable, including snow and a lot of rain.

What you want to see in Berlin would determine how long you want to stay. I spent two days there many years ago. It was enough time for me to see what I wanted to see(my favorite was the Pergamon Museum which may not even be open right now) and I've never gone there again. On the other hand, I would not have chosen to go to Neuschwanstein but went because my daughter wanted to see it and loved our stay in Fussen. The "castle", was, as I expected, just a palace. To me "castle" is the term for a structure which was originally built for defense purposes before the age of gunpowder.

Why Dusseldorf? It's nothing special compared to so many other amazing options. Cologne was also totally destroyed in WW2 and rebuilt, so it doesn't have that old, quaint feel (although the cathedral is amazing).

We love Berlin, Munich, Nuremberg, Passau, Regensberg -- maybe tell us what you are looking for in a town/visit? And know it takes lots of time to change towns. I'd book MultiCity - fly into one and out of another, don't circle back. Will you be using only the train?

For 10 days, I'd do 2-3 towns and do day trips. Expect a rainy visit.

Cologne was also totally destroyed in WW2 and rebuilt, so it doesn't have that old, quaint feel (although the cathedral is amazing).

I really enjoyed my visit to Cologne, notwithstanding the lack of a "quaint" feel. The cathedral itself is worth a visit, but the city is really interesting, and I loved my 3 days there. I don't think it would be worth a large detour, though, so a lot depends on where you visit. If you decide to settle on the Black Forest and the Rhine, it's definitely worth a stop. But I wouldn't travel from Munich or Berlin to visit there with only 10 days.