My only concern here is that rewarding people in this fashion is optimizing for the wrong thing. Rewarding for “approvals” encourages people to review the easy PRs and skip the hard ones. Rewarding for “requests for changes” encourages people to nitpick PRs for subjective stylistic things. Rewarding for merges is practically the opposite, it encourages people to merge PRs that otherwise could have benefitted from some feedback.
That said, I don’t have any ideas for how to fix this while still retaining the notion of rewarding people, so maybe it’s worth doing this anyway and just keeping an eye on things to make sure this increases contribution overall instead of just redirecting existing contribution. If this produces more contribution overall it may still be a win, with easy PRs getting reviewed and merged faster (which makes everyone happy) without necessarily slowing down hard PRs.