Can't Upgrade Mail from Catalina to Ventura

The simple version is I have an older iMac that I've used for years and I can't upgrade the Mail data to Ventura on my new M1 Mac Mini. I've tried pretty much every avenue to do this, including a few long conversations with AppleCare reps who were very helpful.

I have tried Migration Assistant. First I connected the two computers over my LAN and tried to migrate my apps from one to the other. I never saw a clear indication data was being transferred. I got the "Cylon eye" effect, where a blob goes back and forth on the status bar and it never started at the left, with the black line increasing across the line to the right. I tried connecting the two Macs directly by Ethernet. No joy. I'd be willing to use a lightning cable, but I don't see any version of that which will connect to the 2013 14,2 iMac on one end and the M1 Mac Mini on the other. They just do not talk to each other.

I tried using Time Machine backups, then saw that Time Machine has not done backups in a week - WITHOUT any notice to me that it was failing to make backups on my 2 Time Machine systems on my LAN. So I took a large HDD I had in a USB case, attached it to my old iMac, set it up as a Time Machine device and tried to back up as little as I could to get my apps and the library. Still took about 24 hours to make that new backup. I ejected it, handled it properly (like turning off the USB drive container and so on), put it on the Mini, and then used Migration Assistant on the Mini to import all my apps so I'd get the full mail data.

It imported a lot of apps - including ones I really didn't want to copy over - but did NOT import thousands of email messages in mbox format on my old iMac. I have, including my iCloud and Google IMAP accounts, somewhere around half a dozen email accounts. Most are POP3/SMTP. I find at least a few times a year, I have to search them for old information dating back before 2010. (Old names, phone numbers, title closure companies we used in various cities - important info in those messages.) I also find I'm frequently going through emails to get company info from when our house was built to buy new things from the suppliers or to call with questions. The bottom line is I need those thousands of emails from as far back as the 1990s available to me. Migration Assistant did NOTHING about them. They were gone in the new Mail program on the M1 Mini.

I've tried just copying without any useful result. Mail uses different versions of mailboxes. Back in 2013 or 2014, I could copy the ~/Library/Mail/V2 directory tree over from one Mac to another and it worked. Now the iMac has the mail in ~/Library/Mail/V7. Mail on the Mac Mini uses ~/Library/Mail/V10. I've experimented by copying the entire ~/Library/Mail directory structure to an SDD, then copied it to ~/Library/Mail on the Mini. I tried this by not touching V10 and by renaming V7 to V10. One of them gave me mixed results.

I also tried exporting all the mail boxes from Mail on the iMac and importing them. For the amount of emails and attachments, I was amazed at how quickly those were exported! However, when I imported them, my folder structure was gone and ALL emails were marked unread. The rules were copied over, but all unchecked. There are emails I've put in folders that don't follow the rules I've created. If I just applied all the rules to every email, I'd get things misfiled.

I really need to move all the emails, the rules, the folders, and actual mailboxes, over to the new Mini and preserve what I have now. But I can't think of a way to do this that I haven't tried.

I'm beginning to think the only way I am going to be able to get all my emails back, and filed, will be to write my own script and do any work I need to so I can convert from Apple's mbox format to the standard one, then copy all the emails to my own server and either just keep using that separate server on my LAN and using MAIL in only IMAP mode or to allow it to read in all my emails from my server. It just does not look like Apple provides a way to upgrade email if the version is too old.