Software Update fix for any Catalina unsupported mac
Check here for more info: https://forums.macrumors.com/threads/macos-10-15-catalina-on-unsupported-macs.2183772/post-28320558
this allows to download a full BigSur Installer even from a patched Mojave HFS+ installation: https://forums.macrumors.com/attachments/bigsurotaswufix-app-zip.943087/
Check here for more info: https://forums.macrumors.com/threads/macos-11-big-sur-on-unsupported-macs-thread.2242172/post-28740283