Ask engineers to open PRs -- even if they're WIP -- early. That way other engineers can check in and unblock those who may be stuck, or who are shy about being stuck. (This is especially important for anyone less than Sr 2.)

Doing this also keeps people focused on the task with less procrastination. No commits in 3 days... why? In my experience this is the best way to keep people moving while also increasing visibility into technical code at all stages of the process.