disable siri permanently in big sur
One way to do this is via MDM policy.

https://support.apple.com/guide/mdm/siri-restrictions-mdm993369d06/web

If you don’t have an MDM, I recommend trying JAMF now for free since it is supported and free for the first few devices. You can fine tune things like the menu bar as well.

https://derflounder.wordpress.com/2016/09/20/blocking-siri-on-macos-sierra/

You might try these profiles as well but that operation can be fiddle without the MDM setup completed.