How do you champion continuous improvement activities to reduce waste? (Lean thinking)
Some degree of re-work, especially when it results from experience and validated learning, is both expected and beneficial in Scrum. I'd suggest that a better ambition in a complex environment is not to reduce waste, but to learn how to build the right thing at the right time.
There is this neat thing in Scrum called the Sprint Retrospective. The purpose of it is for the Scrum Team to evaluate how they work together and to offer opportunities to improve.
But as @Ian Mitchell said the iterative process of experiment, evaluate, adapt will lead to some necessary levels of rework. If you are trying to be a Lean purist, you will have some difficulties. But there are things that can be done to help reduce waste. For example, fine tuning of your CI/CD pipeline (if you have one) or introducing more automated testing that can be done during the build process. Those help to eliminate waste and are beneficial in an iterative process.
The most important thing you have to remember is that "you" don't champion the improvements. The entire team works together to implement the improvements that make the most sense. With that I refer back to my opening statement.