Hi Andrew,
Currently, we can provide limited support on such 3rd-part migration tool.
In our forum, we recommend customers use “cutover” migration to move all the mailboxes from a hosted Exchange service. However, we need to double check the requirement of cutover Exchange migration are met. We need an admin account have the full permission for all the mailbox. For detailed information, you can see the article Migrate all mailboxes to Exchange Online with a cutover migration.
If you couldn’t get enough permission for migration, PST file is another option. You can export all the mails to a .PST file then import it to a corresponding mailbox on Exchange online side through Outlook.
Thanks,
Frank