Help with 365 migration please

Can anyone help me with this Migration please.

The tenancy has existed for some time and all users have had a business standard license to enable them to use Teams and Office 365 for some time. This means that mailboxes were created automatically but these have never been used.

AD Sync was in place.

I am trying to migrate from Exchange 2016 to 365. I have:

• Checked each user has an e-mail alias in Exchange @leonardgray.onmicrosoft.com.
  Configured Outlook Anywhere on my on-premises Exchange Server.
  Enabled MRS Proxy on your on-premises Exchange Server.
  Use the Microsoft Exchange Remote Connectivity Analyzer to test your connection settings.
  Used the Outlook Anywhere (RPC over HTTP) or Outlook Autodiscover tests - it is showing no issues.
  Checked the permissions on the account I'm using to migrate (domain admin and has FullAccess and assigned WriteProperty permissions).
  We're not on Exchange server 2007 so no need to worry about unified messaging.
  The domains are verified.
  The users are created licensed.
  I have the CSV with the list of users.
  I've created a variety of migration endpoints and it verified as it saved and saved successfully.

I initially tried to use a cutover migration as it is what I have used successfully in the past. This failed, when I researched the error the articles suggested that it was because AD sync was in place. So I turned off AD sync. The cutover migration continued to fail.

The articles suggested that a Staged migration was a better option for this scenario. This failed with this error:

Error: MigrationTransientException: Failed to update the on-premises mailbox with the target address (SMTP:email address removed for privacy reasons). The mail sent to the user will not reach user's hosted mailbox. The error might be due to account credentials used for migration not having enough permission to write data back into the on-premise AD. Please ensure that the account credential has Domain Admin privilege. --] We weren't able to connect to the remote server. Please verify that the migration endpoint settings are correct and your certificate is valid, and then try again. Consider using the Exchange Remote Connectivity Analyzer (https://testexchangeconnectivity.com) to diagnose the connectivity issues.

I did some more reading and found an article that suggested that Staged migrations can only be used from Server 2003 and 2007. So I decided to try a Remote Migration. This one failed with the error: Error: TargetUserAlreadyHasPrimaryMailboxException: Target user '902b455c-5986-47c7-9988-bb7282df52b5' already has a primary mailbox.

The articles about this suggest I need to delete the data in homeMDB and homeMTA. The data in homeMDB looks like a pretty important link between Exchange on prem and Active Directory. I'm not particularly comfortable with just deleting this.

Can anyone tell me what I'm missing?