MOVING IMAP MAIL TO LOCAL DRIVE

Select the messages in Mail.app > Message > Copy To > On My Mac > some local mailbox (goes to ~/Library/Mail -- zip and archive that folder if necessary). If that succeeds, optionally delete mail from IMAP.

Create new Mailboxes in the On My Mac section. Move messages into those mailboxes.

not an easy answer, takes some homework.

start here for ideas:

https://www.nucleustechnologies.com/blog/backup-apple-mail-emails-to-external-hard-drive/