I make it work in the end. I guess the launch services were in some undesired state. After running restarting them according to https://discussions.apple.com/thread/251735453?sortBy=rank the registration process started working. Thank you!

Forgot to mention that the trick with applicationWillFinishLaunching works nicely to avoid crashes when registering as securityagent. However my CTK extension is still ignored by the logon screen after restarting the machine

Thanks for your feedback. Can i ask more about the registeration process for the CTK extension? Am i supposed to even use the securityagent approach? It does not seem to be working very well (anymore)... see e.g. https://github.com/frankmorgner/OpenSCToken/issues/37 is there some other way to enable the extension before login? e.g. after restart? For example the built-in PIV CTK extension works well.