Merger: Migrating Tenant To New Tenant (With A Catch)

Hi, I need to migrate 2 office 365 cloud only tenants. However, the "Catch" is that the destination tenant is very large and has lots of additional azure/o365 features implemented and the source is very small and is bare bones basic BUT the source holds the new domain name for everyone. So the source has to be the tenant that currently uses the final domain name that everyone on the destination should eventually use. (the destination users dont need to be switched to the new domain right away, they can stay on their old for now)

If it was just a matter of the destination having the final domain name, i could mvoe the mailboxes and onedrive, and then just assign them the new domain and it would be relatively straight forward.

Howeverr in my case, I feels like no email downtime (lost emails) will be very difficult.

I have read thrrough the microsoft migration tools but am not entirely clear on the most important part, which is when the mail flow is moved from t he source to the destination. Also, I wont be able to setup new usernames on the destination until the domain is moved, further complicating things.

I am open to 3rd party migration tools, I have seen a few out there but have spent most of my time trying to understand the microsoft native system up until this point.

Anyone done something similar who can give some advice? Thanks!