Mail 'On My Mac' folders not transferring to new Mac

Greetings Apple Discussions,

My question pertains to owning two Mac computers, both of which are Minis. An older M2 and a new M4.

The M2 is the one I've been using ever since it came out. In my Mail app I created several mail boxes/folders that are located within the "Import" area under "On My Mac". (See attached screenshot.) I have since purchased a new M4 and would like to have those same mail boxes/folders within Mail on my new M4. (See 2nd screenshot.) My issues is when I click on the "Import" mail box/folder on my new M4, I see all 10,000 of my saved emails...but not the mailbox/folder structure like on my M2.

Being a retired Apple Certified Tech, I have (or at least had) a very clear understanding of how the OS file system works. With that said, I need help with finding the proper file(s) to copy from the older M2 to the new M4 in order to make my Mail "Import" section have the same folder structure as on my M2.

Here is what I've tried so far to no avail:

I've tried copying the ~Library/Mail folder from the M2 to the M4, along with two Apple Mail .plist files found in ~Library/Preferences. (com.apple.mail.plist and com.apple.mail-shared.plist) Neither of these moves have created the folder structure into my new M4's Mail window.

Any help would be greatly appreciated.

Thanks, Dale.