Far be it from me to defend ME2, but I think the fundamental issues with the overall ME storyline are actually nearly intractable.

The major problem is this: the ME series is a very personal scale game. You talk to people and shoot dudes, that's it. You don't manage planets, or fleets, or civilizations. You don't even fly a spaceship. You walk around and shoot and talk to people, then a spaceship-shaped loading screen takes you to the next map where you shoot and talk to people. This, on its own, is perfectly fine. It facilitates bonding with your crew and exploring the galaxy directly and having interesting, personal-scale stories. You, Shephard, are special and awesome and all the cool people want to be your friend. Great structure for a game, 10/10, rock solid concept.

It becomes a problem when combined with two other things. Firstly, the Reapers are not a personal scale threat addressable by the personal-scale gameplay verbs and story themes. You can't shoot them with your space-pistol. Even if you could talk the entire galaxy into uniting against them it explicitly doesn't matter. In game and in the themes of the story nothing you can do addresses the threat. Everything the game is structured around is dealing with the people in front of you, but the Reapers are hilariously impersonal, they eat entire civilizations. Any universe that contains a Reaper is one where your tragic backstory suddenly matters a whole lot less, and your space-pistol matters not at all. It's like worrying about your dinner plans right after a facehugger shoves itself down your throat, except it isn't your personal biomass its but your entire civilization. You're now occupying a very different spot on the food chain, a perspective shift is in order. Conversely, any universe where your personal choices are super important is one where Reapers don't make any narrative sense at all, because they have nothing to do with that, and solving one has nothing to do with solving the other.

Secondly, it's sci-fi and not fantasy. Worse, the first game set up an expectation of at least sort of hard-ish sci-fi. Not really hard, more like jello-hard, but compared to the cream of nonsense soup that is most videogame sci-fi it feels like a goddamn razor blade. The fantasy solution to the impersonal kill-everything demon horde is obvious, you are that special, you get the special sword, and you personally kick their butts. Magic swords and prophecies just work like that, and nobody bats an eye. But you can't do that in even jello-hard sci-fi. Worse, demons don't require explanation. They want to kill everybody, they're demons, that's what the do. The explanation for a demon is the word "demon." But omnicidal robots, although nearly exchangeable with demons, are still machines. Somebody built them for some reason, and the player base is going to want to know why. So now you need a good explanation for the regularly scheduled galactic annihilation. You also need some sort of plot device off switch, preferably one that feels personal because the entire story is fundamentally personal, and also you can get to by shooting things and talking to something. But if its personal it also kinda runs the risk of making the Reapers feel kinda silly, like what, it just took this one person asking just right to stop the eternal rain of blood? Silly extinct civilizations, why didn't they think of that? Oh, and it needs to recognize how you played the series up to this point, but it cannot lock you out of an acceptable ending because of some seemingly unrelated choice you made three games ago, and you have to be able to get there by being a dirty space hippie who unites everybody or a total hardass who tells the rest of the galaxy to take a hike, and the companions have to matter but also they can't matter because any number of them could be dead.

I don't think this ending exists. There's too many constraints, and several of them are, if not actually contradictory, teetering right on the edge of it.