Your proposal to allow moderated grading for group assignments is worthy of a proposal to improve Canvas via the Ideas and Themes process. Without that, I would use a tool outside Canvas to collect feedback from the graders before the moderator then goes in and applies the feedback inside Canvas.
I'm going to mark this reply as a solution. I realize it does not solve your problem, but our colleagues who have a similar critique will also find the best way to pass on the feedback. There can be more than one solution, and I hope our colleagues here in the Community can share how they tackle this issue.
