Thank you so much for developing Whonix! I use it every day and sometimes send donations.
I just upgraded my host to Debian bookworm and, following your installation guide, added fasttrack and backports repositories, but it seems there is no virtualbox-qt there at the moment (E: Package ‘virtualbox-qt’ has no installation candidate). Is the Kicksecure repository the only place where I can get this package? If that’s the case, you probably need to update your manual: /wiki/VirtualBox#Debian