It didn't give me any peace after all. I did everything all over again.
Downloaded OpenCorePkg-master, had it compiled, created a fresh config.plist from a sample.plist, did an clean-NVRAM, reset the Linux boot entries in the shell, and so on.
The result this time is a working configuration, as it should be.
Now I just need to find the error that caused it to fail before, but at the moment I can't see the wood for the trees.
Thank you very much for your help, it has motivated me.