Browser Cache & Cookies

The most efficient way to clear history and cookies is not while you are in the browser with n-tuple tabs or windows open. In Safari 12, Clearing History : all history will remove all cookies except from the current tabs or windows.

It is best to quit Safari, and then press and hold the shift-key when launching it again. Now, there are no open pages or tabs, and Clear History : all history removes all cookies. Then browser cache is removed per Eric's instructions. Once you have that Developer menu, you can also use the option+command+E (⌥⌘-E) keyboard shortcut to clear the browser cache.

Safari/Preferences/Advanced - enable the Develop menu, then go there and Empty Caches.

You can delete cookies by going to Safari/Clear History. If you don't want to clear your history, go to Safari/Preferences/Privacy/Manage Website Data and delete the cookies there.