Typically, 520 errors are related to slowness at the origin. While your browser may wait a bit longer than Cloudflare (hence the problem seemingly going away when you pause Cloudflare), Cloudflare has a limit of how long it waits for the origin to respond.
Some of the settings you’ve set on your page rule can actually make things worse. For instance, you should not bypass cache on the
/wp-admin/ directory, as it has many JS and CSS files that are cached by Cloudflare by default and need not be requested every time to the origin. I’d just keep Disable Apps, Disable Performance, but increase security level to either normal or high. These settings have little bearings on the origin performance.
In my WordPress installations, I got rid of similar issues by changing a few plugins that were too resource-intensive. I’d suggest you try to measure the performance of your plugin with the P3 plugin, then perhaps finding substitutes for any plugins that might be reasonably expected to cause this.
Another suggestion is that you open your WordPress panel with Developer Tools open (F12 on Chrome/Windows). Some themes and plugins will load unnecessary JS files even when you are not in their admin pages, which can cause slowness for no good reason. If that is the case, you might want to contact the theme or plugin author directly to discuss your findings.
A 520 error occurs when the connection started on the origin web server, but that the request was not completed. The most common reason why this would occur is that either a program, cron job, or resource is taking up more resources than it should causing the server not to be able to respond to all requests properly.
Review the Quick Fix Ideas in this Community Tip for troubleshooting suggestions.