In the Gmail search box, type …
-label:inbox -label:sent -label:drafts -label:trash -label:notes
This should give you all your archived messages (excluding the emails in the inbox, sent, drafts, trash and notes).
You also probably want to consider changing the IMAP action after an email is deleted from archiving the email (default) to sending the deleted email to ‘Trash’.
I prefer the latter, since it places the deleted email in the most sensible place - ‘Trash’. And deleting the email from ‘Trash’ now permanently deletes it and archiving does the obvious - archive. Keeping your ‘All Mail’ much cleaner.
Ps: If you have other custom labels you’ll need to add them in there as well.