Preserving permissions during EXO migration

Hi,

Can you help me understand the outcome of preserving the permissions in our scenario.

Exchange Server 2016 (soon Exchange SE) in a hybrid with Exchange Online.
We are moving 75% of the mailboxes to Exchange Online.

What ways will preserve or break the full-access or sendas permissions?

I guess best way would be to migrate both the user and the shared mailbox at the same time in the same batch to keep the permission?

If we migrate the user in batch 1 and shared mailbox in batch 2 will that preserve/break the full access/send as?

If we migrate the shared mailbox in batch 1 and usermailbox in batch 2 will that preserve/break the full access/send as?

If the permission is linked directly on the shared mailbox or via a security group is there a difference?

Thanks!