I have to change the layout EVERY time I open Thunderbird.

My guess is the prefs.js file is being interrupted on shut down, possibly from an antivirus or VPN. And it may even be that the prefs.js file is set accidentally to read only I do NOT have a solution, but only a problem definition, Click help>troubleshootinginformation, scroll to 'profile folder' and click 'open folder' . You will be in profile. If there are several copies of prefs.js, such as prefs-1.js, etc, that confirms the problem.

The layout views Classic, Wide or Vertical are stored in the prefs.js file in

user_pref("mail.pane_config.dynamic", 0); 

where 0 means "Classic". I don't know why the Classic doesn't stick in your Thunderbird but you might try the following workaround:

open a simple text editor, copy & paste the above-mentioned user_pref and save the file as user.js Move the user.js file in your Thunderbird profile folder. Read here how to find the profile folder https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data