As to whether you should do the citadel dlc before or after the end, I'll just say this: when I finished ME3 for the first time, I felt like an empty husk of a person. Like I could never enjoy anything ever again, especially anything Mass Effect related. However, after I played the citadel dlc when it came out a year later, I actually had hope for Mass Effect as a franchise and no longer had a bitter taste leftover from the ending.
So my vote is definitely for doing the citadel dlc after completing the game as a bittersweet reward for enduring the ending.
edit: also the tone of the citadel dlc is like whacky off the charts and would result in whiplash going from that to the final missions, it's much better experienced after the game.
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