Export from Mac Mail without folders

Hi,

I am trying to archive selected emails from gmail as they relate to an ongoing legal discussion (and I don't trust google).

I have used google takeout to export mails from three separate gmail accounts and imported these into mac mail on an old mac mini (running Monterey). The import worked but created a bunch of folders called categories/personal, categories/updates, IMAP_Forwarded etc.

I spent most of yesterday sorting the mails I want to keep into a single folder per account. I then exported each folder into a separate mbox file.

The old folder structure has been recreated in the mbox files which leaves me with a complete mess to sort out (again).

How can I export to the mbox without the structure?

Pete