What are the benefits of conducting accessibility checks on the design library level vs. after implementing those components?

Post-production rework and remediation are the most expensive parts of accessibility. Any accessibility violations you can plan for and catch before your design goes into production saves you time and money. You can start by adapting the custom components in your design system to the W3C design patterns. Then later, when you design the page, you'll want to consider how they come together with things like focus order and labels.

Of course, it is always worth it to do a post-production audit or check to ensure everything is implemented correctly and to catch anything else that may have been missed in the design phase.