How do I migrate the @me.com email from one Apple ID to another? So my girlfriend has this setup with her Apple IDs, where she has one Apple ID as her primary, with which she is logged into iCloud on all her devices, and she also has a second Apple ID which she only uses as an email account.

I would rather like to streamline this whole setup by migrating the email address of the secondary Apple ID to the primary.

I do know that it is not possible to simply merge the two Apple IDs, but I thought there may be a way to migrate an email address. The emails associated with that address wouldn't even have to be migrated over since I can do that manually within Outlook.
No - Apple doesn’t split the mail accounts or aliases - once one is associated with one account, there’s no releasing it or migrating it or reassigning it. I’ve tried twice to do this with support and it got escalated to engineering both times.

The answer was no - set up forwarding or make a new mail account. Maybe this changes, but Apple treats an email associated with an AppleID in any way as unmovable. The only time I’ve had any relenting from that stance was for rescue and security emails. I argued that then can’t tie my external address to one AppleID since I may choose to move to a new ID for the reasons they explicitly designed. 

If you control the @ domain - you can have some measure of moving, but I wouldn’t assume you could move it on to a new account - just that you could discard it and never use it on the new account if we’re talking about the AppleID and not a secondary mail.

If in doubt, get in writing from Apple before you change that they confirm it will be reversible. That saved me when I got my rescue mail and needed them to fix it like they told me. Without written confirmation, I suspect I might have gotten a no.