Unless it's moved in more recent OSs, it's also available in Safari's "Edit"->"Speech" menu and can be triggered with a KM "Select or Show a Menu Item":
Worth trying before you start digging into other methods.
Are you on Ventura? A lot of submenus have become dynamically-populated in the latest OS and aren't available until you select the "parent" item. Try this workround:
You can get the exact menu item by using the "Menu" dropdown in the action itself and navigating through the application, top-level menu title, sub-menu name, etc. KM will then fill in the dialog for you.
Not just possible -- really easy! KM has a Menu condition, and you can use the fact that Safari's "Stop Speaking" menu item is only enabled when speaking is active.
In case you did end up needing the two-step activation, here's the toggle version: