To disable your AD Sync with Microsoft 365 follow the steps below:
Connect to Microsoft 365 PowerShell
Connect-MsolService
Disable the AD Sync
Set-MsolDirSyncEnabled –EnableDirSync $false
This could take up to 72 hours to return the result you are expecting. To check the status use the command below
(Get-MSOLCompanyInformation).DirectorySynchronizationEnabled