It's definitely true that it's exponentially more difficult to find a tour operation that does anything before April, since in Germany March oftentimes still sees snow on the ground (perhaps not this year). Will you have access to a car? It wouldn't be a bad idea to do it on your own. You'd miss the knowledgable guide but at least you wouldn't be tied to a schedule. Luckily, Munich is situated really well as a "hub" of sorts. You have great destinations going out in all directions, be in Fussen southwest, Nuremburg north, Augsburg northwest, Mittenwald or Garmisch to the south, etc etc. If you don't have a car, I might recommend using a rail service to one of the destinations (picking which destination would be the hard part). One thing to keep in mind that the famous
Neuschwanstein Castle is near Fussen.