I found Kanban a lot better for this. You have a sorted list of tasks, and you can only work on a small amount of them at once. A lot less time is spent estimating tasks to fit them in a pointless two week bucket.

Backlog grooming can be done asynchronously, constantly. The PM can push a task to the top without making the developers work harder this sprint. They can go faster or get delayed, because they don't have to meet the arbitrary requirements of a sprint.