Lost mail in OS Catalina upgrade

Several people have reported lost or blank e-mail messages after updating macOS to Catalina. I lost 50,000 messages. Here's my solution, which requires a back-up of messages made before the Catalina update.

1. Restore the folder V6 from User/Library/Mail to the desktop.
2. Open Mail if it is not already open.
3. Create a new mailbox where you will put recovered messages so that you can review them before storing them with other messages.
4. Select the command "File" - "Import Mailboxes  … ", then select "Apple Mail" as the format.
5. Look in the folder V6 on your desktop for a mailbox that you want to restore, select it, then click "Continue". The import will start. A message will tell you when the import is done.
6. After the messages have been imported, the messages will be in a new mailbox called "Import". Within that folder, they will be either in the simple folder structure that you would expect or in a multi-level folder hierarchy. If they are in a simple folder structure, click on a message in the message list, press command-A to select all the messages, and drag them to the folder that you created in step 3. If they are in a multi-level hierarchy, option-click on the triangle next to the folder called "Import" to expand the hierarchy. Command-click on all folders named "Messages". Click on one of the messages in the message list. Press command-A to select all messages in the message list. Click and drag the messages to the new mailbox that you created in step 3.
7. Progress in moving the messages will be shown at the bottom of the folder list. After all the messages have been moved, delete the "Import" folder by right-clicking on it and selecting "Delete Mailbox  … "
8. Repeat steps 3 to 7 for other mailboxes.

Methods that did not work for me:

• Re-building mailboxes.
• Deleting envelope files in folder V7 and forcing Mail to re-index V7 messages.
• Temporarily removing folder V7 and forcing Mail to re-convert messages from folder V6.
• Restoring a Time Machine back-up from within Mail. Mail crashes.