After you've completed onboarding, one immediate value you can create is making life easier for the next person. Improve the onboarding process, support more junior engineers, and continue building relationships.
Onboarding is fundamentally about building trust. (Remember the trust meter from the beginning of the course?) Trust is the currency through which you grow your career, and you’re always gaining or losing trust. Trust is how you get influence, which is essential for further career growth.