Hi @Jon Mercer ,
Are you using Migration Manager in SharePoint admin center? Have you tried to use SPMT? You could try to use Microsoft SharePoint Migration Tool (SPMT) as a workaround. The SharePoint Migration Tool (SPMT) is a free and easy to use migration solution to help you migrate content from on-premises SharePoint sites to Microsoft 365.
You could Install the SharePoint Migration Tool (SPMT) here. Then create a migration task with SPMT.
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