I wish to send you a Big Thank you, MaLd0n!
I got it working easily with the help of your files and guide Basically I took my working OC folder for Catalina, updated OC and Kexts with your scripts, copied the updated files to my "old" Catalina OC folder, and finally changed the config.plist with the two NVRAM entries.
I have Big Sur on an old external SSD connected by USB, and I will probably test to clone it to an internal SSD later today.
I haven't tested if all hardware works, as I just got it installed a couple of minutes ago, but audio works, Eehernet works, and it finds my Intel 530 graphics as well as my Radeon VII card. I will test USB 3.0 later.
Again, many thank's for all your help!