You should make with createinstallmedia a stock USB BigSur Installer (that doesn't contain telemetry), then booting from it open its recovery terminal:
Code:
diskutil list, diskutil mount your BigSur diskXsY (even if I guess it's already mounted)
cd /Volumes/YourBigSurLabel/System/Library/UserEventPlugins/
rm -R com.apple.telemetry.plugin
If you want an easy way to boot the stock Installer without nvram or what else, after you have an USB BigSur Installer, show all hidden files, and edit this file /Volumes/USBInstallerBigSur/Library/Preferences/SystemConfiguration/com.apple.Boot.plist
Code:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Kernel Flags</key>
<string>root-dmg=file:///BaseSystem/BaseSystem.dmg -no_compat_check cs_enforcement_disable=1 cs_debug=1 amfi_allow_any_signature=1 amfi_get_out_of_my_way=1</string>
</dict>
</plist>