Would you consider adding a new setting for your plugin, which allows it to be placed as a submenu underneath the Tools menu in the WP admin pages? (So the administrator can choose where the menu is located - either where it currently is, or underneath Tools).
I'd prefer this myself because I want to keep my top-level menu as clutter-free as possible, and because for me, a plugin like GTmetrix really falls within the category of a WordPress tool (ie. for testing page scores).
When it's under the Tools menu, it could use tabs for its own three subpages (Tasks/Schedule/Settings), just like many other plugins use tabs via the default WordPress tabs styling.
What do you think?