After this hell of an update (waiting time, SafetyNET CTS fail, Bug in C20 Rom preventing update to C21 without full wipe, etc.).. there is finally a workaround.
Kudos to @mcboums and @ruszok for the solution and the modified C21.zip.
See here and here.
Now I'm on KB2003 EU C.21 OOS12.1 with...