Hi @Roger Roger,
Thank you for posting to Microsoft Community.
Based on your description, I understand you want to create a new unified group by the command you offered.
However, you may need notice there are no UnifiedGroupWelcomeMessageEnabled parameter match New-UnifiedGroup, the rest part of the command could work correctly as below.
You could use Set-UnifiedGroup to disable UnifiedGroupWelcomeMessage setting as below.
You could refer to Set-UnifiedGroup (ExchangePowerShell) | Microsoft Learn for more information.
Hope it helps and if there are anything else you need help, please feel free to contact me.