I am curious what sort of situation you have in mind that is feel good, while morally grey/black? I could not think of any when I formulated my post. At least I would say they appear very tightly coupled from all feel good examples I have in mind.
Maybe I should emphasize that I here are talking about the surface obvious elements of the ending. For instance starwars has a feel good ending, despite it involving vandalised decades worth of work, and killed thousands of people. It acheives this by putting front and center the focus on this being the destruction of a weapon in the hands of someone that happily just used it to kill millions of innocents.
And I think it is this kind of surficial moral good ending players of D&D should reasonably expect. After all once you start looking deeper into the morality of killing sentient beings, basically all published adventures quickly falls appart..
I don't actually consider the destruction of the Death Star a morally ambiguous ending. Anyone who died on board the station was an active combatant engaged in a (civil) war, who had participated in
outright war crimes (the destruction of Alderaan, an
explicitly civilian target.) Preventing the commission of
even more war crimes by destroying the Death Star is not some sort of horrible mass murder. It is a terrible loss of life, and the deaths of the crew absolutely should be mourned (because all deaths are tragedies!), but it is not something to beat oneself up over.
I would consider it a "feel good" but morally-questionable ending in any of the following cases:
(1) A very bad person, who has done bad things and gotten away with it, and whom the party strongly and justifiably dislikes, gets punished for crimes they
did not commit. The PCs enjoy watching them get their comeuppance. This is morally wrong--no one should be punished for deeds they didn't commit, even if they have done other wicked things--but it is almost certainly going to be
satisfying, hence, feeling good.
(2) The players have successfully stolen something very valuable without getting caught. Taking something that doesn't belong to you is morally wrong, yet such heist-type stuff is often thrilling, exciting, and deeply satisfying if you can pull it off and get away with it (consider
Ocean's Eleven.) This is especially a "feel-good, morally-questionable" ending if the stolen item(s) weren't themselves ill-gotten gains or the like, but rather legitimate goods/purchases/etc.
(3) The party successfully starts a war between two rival powers, ensuring that those powers won't team up against their allies. This is less blatantly "big moral wrong" than the previous two, but it's still very morally questionable. They will have intentionally caused a lot of unnecessary death (among people who
aren't massive war criminals, unlike the Death Star crew) for their own benefit. That benefit might be an overall good thing, but it's still a
very morally-questionable method of achieving that end.*
(4) Basically
any game where the PCs are actually evil or doing the bidding of actually evil employers. Whatever victories they get, whatever things make them "feel good," it's pretty much definitionally going to be morally-dubious at best. These are of course rarer, since a lot of DMs don't run games with evil characters, but it's a relevant example.
(5) Resurrecting a beloved dead friend/lover/family-member/etc., even though it means doing something nefarious. Relatively simple, that one. Getting back the person you love even though it means a deal with the (literal) devil? There are a lot of people who would take that without a second thought and be happy about it, but it's clearly morally dubious.
*You can see a reversed version of this (killing someone innocent, albeit unpleasant, in order to ensure that their people
ally with the "player character" allies) in the
Star Trek: Deep Space Nine episode "In the Pale Moonlight." TL;DR: The station commander recruited an ex-spy to get the Romulans to help in the war against the Dominion. It works...but only because a Romulan senator and his two bodyguards, all innocents, were murdered by the ex-spy. It is a genuine victory for "the good guys" (Sisko himself says that much) that the Romulans are now helping, but it was won through blatantly immoral means--and Sisko finds that he can live with his guilty conscience, somewhat to his surprise.