Basically install mailcow, set the domain to the same as mailu.
Set SPF record to also include your mailcow server.
For each user mailbox/alias address create a transport map which is pointing to the IP from mailu server.
set the MX record to mailcow.
Now every mail should be received by mailcow, and forwarded by the transport map to mailu.
Create a test user on mailcow and check if you can send mails.
Then crate user mailboxes on mailcow and migrate the users with imapsync.
Once the user is migrated, delete the transport map for that user. Now he receives mail in his mailcow mailbox.
So you can migrate users in bunches, practically without any downtime.