Oh, it should absolutely be the team's decision and you're also totally right that Kanban requires a more mature team. People indeed need to be able to recognise and ask for help when they're stuck (which means being vulnerable, but also simply being able to formulate the right questions). People also need to be able to give feedback to their team members when they feel or see that someone is struggling or not delivering enough.

To facilitate I always have some form of retrospective in my teams, even when doing Kanban. Sometimes only once every other month, sometimes every two weeks. Highly depends on the maturity of the team and customer.