Need a better way to save emails

The simplest way to save/retain emails for future use would be to Export to MBOX. Select a folder in Mail, then do Mailbox > Export Mailbox. That will create an MBOX file (on disk, outside Mail) that contains all the emails in the folder you selected. It is a copy process; the original emails are not deleted from Mail. You can store MBOX files anywhere you want to.

Note that Export to MBOX does not support nested folders, so be sure that any folder you export does not have any subfolders in it.

You can retrieve emails from an MBOX file by importing it back into Mail. Imported MBOXes will still appear under "Import" but each MBOX will be imported into its own folder.

You can even use the MBOX technique to migrate your emails to another email app.

Purchase 3 separate reliable External Drives ( Referred SSD drives ) each 2 1/2 Greater Drive Capacity than your Internal Drive.

Setup Time Machine Backup

Backup disks you can use with Time Machine

Setup each of the 2 Drives on a Schedule

It will alternate Backing Up between the drives

The Third Drive to be used by another method like Carbon Copy Cloner or **** Pocket Super Duper to make a Data Clone of the Internal Drive

The Restoring of e-mails is a bit more completed and involves getting to the Hidden V10 File in User Folder >> Library >> Mail on the TM Backup Drive or the Cloned Drive

You mean they are all laying flat inside Imported? Not the original folders inside Imported?

How exactly did you migrate them back?