So I have downgraded my BIOS from F13 to F12. But it doesn't seem to solve the problem. The installer seems to boot but no video signal is outputted from my RX5700XT. I even tried to check if the video is outputted through the onboard HDMI but no success. I had the same problem with Catalina there I needed to add a acpi Patch. I have added it and still no signal.
Also I am using MacPro1,1 as SMBIOS.
Edit: alright I remembered that I had the problem with Catalina as well, a little research helped and I forgot about the
bootflag.
Now Big Sur installer is booting. Cheers mate
Will keep this post updated if install will be successfull
Edit 2:
So the Big Sur installer always crashed before the reboot, after the copying process. I tried it over and over again with different kext's and config. But always an error was thrown: "service exited due to SIGKILL diskarbitrationd"
I read about the solution people tried during the beta of Big Sur. I assigned an external SSD to install macOS via vmware fusion on it by following
this tutorial (German). Big Sur was installed successfully, I put maecsa EFI on the external SSD. Rebooted set the start device to the external SSD, OC was loaded instantly and Big Sur booted. Now I am using the migration assistent to copy the files from my working Catalina enviroment to Big Sur.
Very weird workaround, but I hope it will work.
Edit 3:
It has worked! Big success!