After the Grim Reaper has guided countless people to the Afterlife, a suitable replacement must don the cowl, so Death can finally rest in peace.

This is a good setting but it doesn't describe the story conflicts.

Is the new person happy about the job?
Does he have to die?
Does the GR want to retire?

All that can and must be in the story. Not your story, but here is an example

"The Grim Reaper must convince a father to leaves his family and become the reaper's replacement"