I went as far as creating a new user account, A different user on the same machine has the same setup (3 mailboxes),
Did you create a new windows user account or a mailbox? Did this different user configure his accounts on this machine with a same windows user profile or a different windows profile?
Did this issue occur only recently? If so, have you performed any specific operations before it(such as install any add-ins or programs)?
In order to avoid the interfence of COM add-ins, it's suggested to restart your Outlook in safe mode when the error prompt to check if you could work normally, if you could open it in safe mode, the issue may be related to any com add-ins, please try to disable any COM add-ins to check (File>Options>Add-ins>COM add-in, Go>uncheck it)
If that's invalid, for Microsoft 365 user, we supply the SaRA tools to troubleshooting the "Outlook won't start" issue, it's suggested to download it to check if it could work for you.
If your issue has any update, please feel free to post back.