Once again 1000 times thank you. The upgrade worked fine with your proposed steps and I am in Catalina.
However, YouTube videos do not load at all, QuickTime can't play local video files (takes about 3 minutes to load the frame if I skip to some point in the video, but it still won't play the video) and Logic Pro is not outputting any audio.
is there something I did wrong during the update or something I am missing?
Interesting to know for others doing the update: I had to click the "preinstall boot" partition twice during the installation project. And now I have my windows, FileVault, recovery and normal boot again in my boot menu.