Your idea sounds exactly like DFO's grading system with dungeons, which was a good system for how the game was designed imo. I've had similar thoughts but, like others here have said, trying to calculate mitigation, healing, and mechanic resolutions to a grade would just make it very difficult to implement fairly. Especially with healers since their grade will essentially be based off of the group's performance (passing up dps to heal/rez someone failing mechanics and vice versa).

Although, one metric I haven't seen mentioned so far is completion time. Generally the quicker you complete a duty, it should correlate with better group performance would it not? In the case with first-timers, this metric could be waived with the current reward bonus we have.