OK, threw out the plist. No difference.
I followed your above link, and it looks to me as if Glenn Leblanc is correct, because I just looked at ~/Library/Prefs/com.apple.mail.plist on my MBP (also running 10.15.7), and it was last modified in April 2018, which pretty much confirms that it's been supplanted. (I confirmed that by deleting mine and restarting without the file being recreated.)
I suggest that you follow these instructions:
It's actually in the Containers folder of your user library: ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist
However, if your going to reset the Mail defaults, you would be better to delete the com.apple.mail folder, and not individual files within.
If you have one in the ~/Library/Preferences/ folder, delete it. It won't be replaced.
Container folder path is:
~/Library/Containers/com.apple.mail
Quit Mail, move that folder to the trash your desktop, then open Mail. You will need to reset your defaults. If something goes wrong, you can always put that folder back and replace the new one that was created. (Emphasis added)
The only thing I'd add is that the suggestion that you restart your MBP
immediately after moving that folder to your desktop...before reopening Mail.
