I successfully installed Big Sur on Dell Precission 3510 (6th gen. i7) using OpenCore.
I followed this video:
https://www.youtube.com/watch?v=jqg7MX3FS7M
and this guide here:
https://dortania.github.io/OpenCore-Install-Guide/config-laptop.plist/skylake.html#starting-point
Pay attention on those settings that are intended only for Dell. For SMBIOS, use MacBookPro13,3