The shiftcommand\ keyboard shortcut is used to toggle the Tab Overview option in Safari. This can also be accessed via the View > Show Tab Overview option via the menubar, or from the Show/Exit Tab Overview icon in the Toolbar.
As you've found, this only displays a visual thumbnail of all tabs from the current window, and not all tabs from all Safari windows.
Unfortunately, there's no modifier key you can use to enhance this shortcut, but you may find an Extension that does what you want. Extensions don't necessarily work for all versions of Safari, so you may need to experiment a bit to see if one exists that can do what you want.
You can browse for Extensions in the Safari Extensions Gallery.
Finally, you may want to submit some feedback to Apple requesting this feature. For example, you could suggest that including the option key in the shortcut (i.e. pressing optionshiftcommand\) works to display all tabs from all windows.
You can submit Safari feedback here.