How to move large amount of mails/mail boxes in Apple Mail between 2012 and 2020 intel iMacs?

To avoid me wasting your time please note that I will not change my current mail organisation and I need all my mail on my new iMac.

iMac 2012 to 2020

I’m in the process of switching between two 27” Intel based iMacs: 

-from a late 2012 model running Catalina 10.15.7 with a 3 TB fusion drive

-to a new 2020 model running Monterey 12.3.1 with a 500 GB SSD.

External backup

I have backup of everything from the 2012 model on seven external disks (yes, I am data paranoid). I also have everything but mails in the cloud (my cloud service does not handle backing up mailboxes well).

Storage size difference

Due to the storage size difference I will move as little data as possible onto the 2020 iMac (around 160 GB including mail). 

High number of mails and mail boxes

I have around 100,000 mails in hundreds of different mailboxes (as stated I want this way of organising my mail to continue) all downloaded and stored in the V7 folder (the folder size being around 76 GB). I would very much like to avoid having to export each mailbox on the 2012 model and import each mailbox onto the 2020 model (but will do it as a last resort) but I am unsure how to proceed.  

No trial and error please

For boring reasons I have huge difficulties getting an overview of these things and I would very much like to not do trial and error. I apologise in advance if I have missed posts regarding this particular challenge. Now the questions:

1) Difference between V7 and V9?

Is there any difference between the V7 and V9 format or can I just copy the V7 folder over physically to the 2020 model and rename it to V9 in the Library folder?

If 1) is a no go:

2) Migration Assistant?

If I use the Migration Assistant (via ethernet) to move data will it move the mails from the V7 format to the V9 automatically? Or will it not move mail data at all? I can temporarily delete all the other data but mail on the 2012 iMac before starting the migration process.

3) Help?

If 2) does not do the trick either - has anybody any idea how to move this much mail data?

Your constructive suggestions are very much appreciated - thanks in advance.

xpan