Is there a method to migrate a single (or several) domains from one MIAB to another new MIAB?
Not automatic that I am aware of.
To do so manually, you would need to create the users/aliases on the new installation and then either copy the mail directories or use a tool such as imapsync.
Is there a method to manual stop receiving mail for just one domain or user?
No, you can’t stop one mail client for receiving mail for one or a couple of domains.; but you can set the TTL for you MX record to - for example - 5 minutes. If you do that step, a couple of days, before your actual migration, you can limit the time of delivering to your “old” location.
You have to setup your new box, rsync or imapcopy the data and change the pointers (after testing)
Yes. For one user, archive the account. For a domain, remove the domain’s MX records from DNS.