@claus i figured out how to do it with marcan's advice, btw. you have to set the boot security mode to like medium/slightly reduced, so it will accept any previously signed OS version, not just "latest" (which requires an online signature that's a certain amount of recent)
@sneak good luck with running your mac offline then. What you are saying is that critizising technology is stupid and naive because it did good things elsewhere. And I am just surprised by that because you are yourself criticizing a lot. (And I think criticizing is the only way to find balance). But be it, lets disagree.