Hello,
Look, we do really want to help you but you don’t want to share the server error logs to check what error causes the issue and don’t want to share your FTP access to allow us to check the error logs by ourself and make the necessary changes to fix the problem if this is related to our theme/plugin. How do you expect we should fix the issue if we even don’t have access to files and logs? 504 error could be caused by a lot of things. Error log helps to understand what exactly is the reason of the problem.
So, please, provide us with server error logs and create a temporary FTP account with access to your wp-content folder only. We don’t need access to any other folders of your installation.
Regards