Need Help Figuring Out Domain Sharing Solution

Hey microworker 

See if the address rewriting service using exchange server meets your requirements:

https://mymicrosoftexchange.wordpress.com/2020/03/13/how-to-share-the-same-email-domain-between-two-exchange-online-tenants-for-long-term-mail-flow-tenant-to-tenant-coexistence-part-1/

If you are looking for paid solutions Quest does provide a coexistence service, you might have to give them a call to discuss your situation where you will not be migrating the users would rather stick with coexistence mode:

https://www.quest.com/community/blogs/b/microsoft-platform-management/posts/coexistence-in-office-365-migrations

Thanks