I'm running the latest Big Sur with CSRUTIL disabled via Recovery Disk terminal session. I'm going to try modding the SystemConfiguration NetworkInterfaces.plist and add the ethernet interface that way. I'll report back.
You shouldn't need to mod the plist. Usually deleting that plist and the devices in the System Information > network pane does the trick of getting the Ethernet connection to show in the preferences pane.
I think it the reason Big Sur doesn't allow you to change the plist is due to the way Big Sur uses SSV (Signed System Volume) on the entire boot volume.
The Gatekeeper tool in Hackintool will only mount a APFS volume in read/write mode. It won't convert the volume to an un-signed/Unauthenticated volume, which it would need to be in order for the plist to be edited or deleted.