About the "abrupt" ending, I get it's a lot of information for the player to take in and an epilogue certainly helps with that. I actually prefer the original ending without the epilogue for stylistic reasons and because the information on the epilogues are irrelevant, I was fine without those infos. But it does add more meaninful dialogue and the overall presentation of facts is more palatable.
Thematically speaking, I don't see how it betrays ME themes, in fact, the entirety of 3 was build towards that ending, that whole thing I alredy said in a previous response about unity and how that ultimately saves the day and breaks the cycle, even if it means that people on the current cycle are screwed.
The game is still about character. I guess people wanted suicide mission part 2? 3 takes a different approach where you actually say goodbye to your friends before the battle because you were probably going to fail, and you almost do - so does your crewmates, they either die or Normandy rescues them before Sheppard goes to the Citadel.
But they were essential in your journey throughout the game and had meaningul character arcs and when you say goodbye to them is heartbreaking.
I guess for you they had to be involved in the final conversation with the Reaper for it to matter? What about everything they did in the game?
Anyway, you're right about Buzz Aldrin (but I liked it)
Here is how I see writing a Bad End, which I define not as a badly written ending, but as an ending where things end badly on at least some major level.
We think of Bad Ends as the thing the heroes are trying to prevent and what would happen if they sat around doing nothing. The Bad End isn't something that can happen, it's that it will happen if not for the heroes. If Shepard doesn't use the Kill All Reapers button then they'll get a Bad End where the Reapers win.
I think this sounds true, and then I came up with a meaninglessly fluffed up esoteric take to make myself seem smarter: The Bad End only happens when the writer allows it, because every action in their universe is controlled by them and if they felt like it they could have the villain slip on a banana peel and die. Writing an ending where good triumphs over evil is magnitudes harder because you need to carefully detail why the upstart rebellion lead by the long lost prince can now take on the rampaging hordes of Emperor Deathmeciatus even though he controls the world, and when he's defeated through the power of friendship you need to explain why and how it worked this time and failed all others. There's no reason the writer can't just draw a straight line for the forces of good to the villain's doom fortress then load them up into a sedan and send them off, maybe stopping for a snack on the way, except the big one: that makes for a boring story. We're not here for the destination, we're here to go on a journey.
Bad Ends are not something that work because they're gutsy and lead to lots of speculation from everyone, Bad Ends are the culmination of the message the writer is trying to convey in creating a story that leads to an ending at the expense of the heroes. They have to matter, to be thematically consistent, to be a natural conclusion to the preceding events, and they have to do all of these so well to overcome the vitriol of a fanbase who get attached to characters and their struggles more than what the writer is trying to say through them. You brought up Breaking Bad, a series wherein a man is dying from cancer in front of an uncaring health care system, turns to cooking meth to care for his family after he's gone, and then gradually realizes he's an asshole addicted to his newfound power and wealth. How else did you think it was going to end? Walter's fall from grace was not just the result of actions, it was what the show was about to begin with.
Mass Effect as a story, boiling it down as simplistically as possible, is about different cultures banding together and overcoming past grievances in the face of an impossible adversary who wants to convert all life into one of them, and the ending to this story involves a single person having a five minute conversation with the singular ruling authority of their enemy, and then deciding what the future of the galaxy should be with no input from anyone else, and this action as originally depicted should have wiped out all life in the galaxy since ME2 tells us blowing up a Mass Relay causes a massive explosion that wipes out the surrounding star system and Shepard just blew up all them, so what Shepard just decided is pointless either way.
The original ending to Mass Effect 3 only lands if the only thing one cares about is if it "makes sense." Sure, it "makes sense" that fighting eldritch death robots is probably impossible and that it would require massive amounts of sacrifice (sacrifices that Shepard is unwilling to make even in ME3 itself since their quest involves rallying support for Earth instead of letting it burn so the rest of the galaxy can figure out a plan), but the problem is that everything the ending is trying to convey wasn't what Mass Effect was ever about.
