I received this email in a response to my inquiry. I was trying to book a tour for mid-April, and couldn't, and thought I was too early. I was really surprised one must book so far in advance. Perhaps make notes in your RS guide. Hopefully my misfortune will be your gain, and you will book as early as you know you'll be in the area. Good luck!
"We are happy to see that you would like to visit our museum and we would love to be able to receive you as our guest.
However, all English / German spoken tours are fully booked
"We do not have a waiting list or plan more English tours.
It’s not possible to make a self-guide tour.
We are sorry to disappoint you, but we hope to see you in the future .
Due to growing popularity, we advise to book months (4 or 5) in advance
As an alternative, you can visit our virtual tour."