Backup Mail Larger than iCloud 20MB Mail Message Limit

Unfortunately this isn’t applicable as the goal is to archive messages that were sent in the past. The goal is to move these messages from Mail mailboxes “On my Mac” to mailboxes on iCloud so they won’t take up hard drive space on the computer. It appears there’s no way to do this that will allow Mail to index the messages - in other words, I can only copy the messages as files and store them as basic files outside of Mail on iCloud, which won’t be very useful except as a backup since I won’t be able to search these messages from Mail anymore. I understand the reason for a 20MB file size limit, but there should be some way to archive these messages on iCloud that allows Mail to index them since other mail providers have different file size limits, which causes these issues.

Unfortunately the information in the file is just basic information (how to move a message, how to delete a message) and the problem is being able to move messages from Mail mailboxes “On my Mac” to iCloud mailboxes when the messages are over 20MB in some way that allows Mail to index the messages. Otherwise, it seems the only way to move the messages from the mailboxes “On my Mac” to iCloud would be to upload the messages as files into the Documents folder, which would disconnect the messages from Mail, making this only useful as a backup rather than to store the messages in iCloud and be able to access them conveniently through Mail, as always. Hopefully a solution to this 20MB limit is implemented - Mail Drop only allows storage for 30 days, so this isn’t sufficient. The storage requirement is long term, but in a way that messages are still accessible through Mail, while being stored on iCloud instead of on the computer’s hard drive.

Is there a way to upload messages as files to iCloud AND have Mail access the messages in some convenient way other than manually opening the messages, which isn’t practical.

Unfortunately that article shows nothing but basic import/export instructions for mailboxes. It doesn’t give information to do anything like create a symbolic link for a mailbox that can be stored in another location, or, to allow files larger than 20MB to be in mailboxes that are stored on iCloud. The only possibilities seem to be what I described in the original post: leave the messages that are larger than 20MB on the computer, which is a problem as these messages take up too much space on the local hard drive, or, simply upload the messages that are larger than 20MB as files to iCloud, which will make them inaccessible through Mail, which is what I’m trying to avoid as this is not a solution.