@KC_SS this can be achieved with using groups with automatic assignment/pages/menus.
You can have a dashboard (page) on a menu for new managers, so that when they log in they see that onboarding dashboard.
Then have a group with automatic assignment rule to assign managers to the group once they complete the course. Then have your second dashboard page assigned to a menu that only has visibility to that group for the managers who completed the course.
Once a manager completes the course they are assigned to the group, and so the next time they log into the platform, they will see the second dashboard.
I think this guide has some more info to help you with setting this up: