How to add new Content Types to SharePoint Online Document Libraries using PowerShell

I would the Get-SPOProvisioningTemplate command and the Apply-SPOProvisioingTemplate. That lists all fields and content types in an xml. Simply update and reapply the xml to the root site of your site collection and the content type is created.

Then for the in the subsites libraries do the same export/import. All you would need is a content type reference included in the list instances. This ensures that you will use the site collection content  types created in the above step.