Creating a style guide and UI components is definitely a solid foundation! I also highly recommend the Design Systems Handbook (https://www.designbetter.co/design-systems-handbook).

The engagement model should be a bit of both:

- Designers who maintain the design system are like the architects, and advocate for other developers to use the system patterns. They should also partner with developers to ensure that the UI components are opinionated.

- You still need to consider your web app's overall UX flows though, so designers are still needed to mock up new features for the developers to implement. Even with a robust design system, there are many ways to easily create unintuitive or messy designs. A designer should be assigned to each team and be in charge of mocking up the team's work and ensuring that the team is properly using the design system.