I’m out of ideas. You have to debug to see where things went wrong. If you are able to debug, place a breakpoint in AUWrapper::initialize and check that it runs thru it without returning an error code.
@MarkusK1 I do not know if that can help but one thing you could try is go to Web - Jamba and generate a blank plugin. Then try to generate the Audio Unit and see if that works. (configure the project and provide the path to your SDK installation (--vst3 option) and run jamba.sh install-au).
I am not saying you should use Jamba for your project, I am just saying that as far as I know I have updated Jamba to work with M1 and I tested on my MacBook Air with Garage band and the plugin was working fine. If this test works it means there must be something wrong with your setup/toolchain.